Phi's 5cp Analysis vol. 2: Sluice
Word of advice, I come from an angle heavily influenced by the 6v6 competitive 5cp meta and some of my feedback may apply less so in pub environments. I realize you are not aiming to develop this map for 6v6, but a lot of the ideas and strategies used in that sort of meta are easily applied and used in pub environments as well. Take all of this with a grain of salt, but don't disregard it just based off of the fact I mapped for 6s and you're mapping for pubs. Here we go.
Ah, Sluice. Formerly known as Mill. It's an interesting map, strongest point would probably be mid. Second feels lackluster and last needs work. As far as I can tell, this map would play fairly slowly in 6s as both teams would choose to run sniper near full-time due to the sightlines poised on the map. There's a possibility that teams could choose to run two scouts alongside a roamer-switched-sniper, but that's theory crafting. Here's some hard analysis.
The Mid Point
The mid seems strong. Soldiers and scouts would have a field day due to the open air and height variance. Medics don't have any safe spots at all and would be constantly in peril during a midfight - potentially promoting very passive team play where medics sit back in the connectors to avoid getting bombed. This might not necessarily be a huge problem (see: process, that mid is incredibly open yet medics have it okay), that's just a warning.
Teams would most likely never choose to push through the lower entrance on the left, as the route to the right is much closer to the mid point and it is much higher up. This entrance doesn't feel useful for anything, it's not useful as a flank because it's entirely visible from both ends and there is no way in hell a combo would be pushing through it (especially into second, as there is a ledge defending scouts/soldiers can hide up in to pop uber early or kill the med):
They're also about the same distance from the entrance choke, so there is no reason to enter through the bottom route other than it being more visible to new players when first playing the map:
The water under your mid isn't a good thing. Only thing it does is give soldiers the ability to perform huge water-boosted rocket jumps that can span the entire point area.
Speaking of lower, this area with the healthkit is essentially a death trap. If teams are in any way competent and holding shack or planks, soldiers/demos/heavies can easily spam out anything coming from underneath without any sort of contest from the pusher. Essentially it nerfs the viability of using that route for anything but traversing after the fight has finished.
An observation: a good mid hold may consist of team members holding on top of shack watching the 2 main entrances and on top near fan, watching water. There aren't any places teams can peek into without being at a large height disadvantage.
The sniper sightline from enemy shack can cover all three entrances into mid while being fairly defended and easy to use: the team pushing into mid can't easily spam out the sniper and he has full reign over anyone peeking in any of those routes. Pair that with soldiers/heavies standing on shack/fan as seen above and you've got a mid hold that most likely isn't breakable with anything but an ubercharge.
There is a sniper sightline from water, usable if the team defending mid isn't in a position to hold forward on shack/fan, that covers a good portion of the mid and could be used to pressure the opposing team out of mid:
All in all, the midfight seems fun, but pushing back into mid after losing seems near impossible without an uber. Not necessarily a bad thing, but the holds are pretty strong.
The Second Point
Second is large, and long. There's a lot of walk distance that is covered by some wicked sniper lanes.
First of all: the sniper perch coming out of lobby encourages the team currently holding second (and being pushed into by the team holding last) to place themselves in very, very passive spots due to the amount of space the sightlines cover. Both of these pictures highlight potential forward holds the team could use if the perch wasn't so strong. In essence, this makes the second point play out in a "is their sniper alive or dead" sort of feel. Not something you should be going for.
Unless you hug the wall, entering from this side basically guarantees you'll be sniped.
Yard is a death trap to attack from, to push into last from (see feedback on Lobby and Last), and it a very large wasteland nobody really wants use. Metal bridge (the bridge leading from point to the cliff) and cliffside (the cliff coming out from the upper shutter) are both wildly more advantageous places to be in almost every situation (note that sniper perch and window covers metal bridge easily)
2nd feels like one of those "pile of stuff sitting on top of dirt", you know, those sort of wannabe-spires like Logjam has. Maybe rethink your detailing for the point, it's very... bland and standard looking as it is. Try to make it some sort of unique stand-out structure.
Here's a big issue with this point: there are only 2 entrances into 2nd, both are covered by
the same sniper lane standing on the point, both lead into the same long approach area. It's not ideal and it feels like it really slows the map down. These walk distances are really, really long.

If you're not holding the second point with a sniper, pushing in is abhorrently easy due to the long approach and no easy way to hold forward against a team at mid. If the other team is running counter-sniper and you're not, you're screwed.
There's no reason to hold this lower ground at all, every route around it is more accessible, higher up, and won't lead you to death by spam and height disadvantage. There isn't any reason to use this route either, yard is useless and the lower ground has no flanks or advantages.
There aren't really any good hold spots for the team holding second besides directly on the point. All the area around the point suffers from being on the same level as the entrances into second, and being terribly far away from said entrances.
Overall, second is too large and doesn't have enough variance in choices of route from mid to second. Yard is pretty useless, and the sniper perch is insane, leading to one side of the area (cliffside and upper shutter) being used a lot more than the others.
The Lobby
Quick observation: these walls have asymmetrical detailing. Not a big deal, just something that could affect gameplay a little due to the indents being in different places.
The lower entrance into last is complete suicide unless the attacking team is rolling. There's no easy way from the lower door to the upper area in the last point area, and all spam from people holding upper (there is no way in hell they would hold lower) will be directed right onto you. Entering from lower shutter on second also leads you to be confronted instantly by anyone holding this staircase, leading to instant death:
Essentially, teams holding last would have a very easy time holding forward in the lobby. A hold might form something like this: main combo sits in upper main lobby, watching upper shutter. The flank holds staircase, watching lower shutter - anything jumping onto sniper perch can be easily destroyed by the combo in upper.
The window in upper lobby also gives defense a huge sightline over 2nd and a huge amount of
uncontested visibility over the team trying to push in. Sniper perch does the same thing, albeit with a different (and arguably more powerful) sniper lane that can be easily retreated from.

The team holding last can easily bounce teams pushing through upper shutter around in the approaching room (since it's the only good route to use for a combo push), since the ceiling is low and the combo has to push through two doorways to enter into lobby:
This shutter doesn't need to exist, it's blocking no useful sightline and it only serves to make using the lower entrance even more of a bad idea by providing anyone unlucky enough to be behind it with no visibility into last until they walk forward and open the door.
Essentially, there's no flank whatsoever in this lobby - lower seems to be intended as a choke but it functions as neither, it's useless to push in from and it's the worst method to push out from. There needs to be some sort of flank in through this lobby, think snakewater or badlands.
The Last Point
Straight off, you should not be able to spam from spawn door straight onto point.
This path up from lower to upper is the only such path of its kind, and the nature of it (sunken back far) makes it only usable as a defender. You need to add some sort of easy method for attackers to bridge the height between lower and upper in last without having to commit so hard to a disadvantageous route:
This far corner seems a little too strong to hold from: sentries can be put here that can not be spammed at or seen unless attackers commit, and a medic can easily hide here free from spam in every way whatsoever. Maybe adjusting the entrances into last could help?
The sniper sightline from this upper area is impossible to contest if you're a defender sitting back: there's no easy way to spam the doorway, and it is impossible to access as defense holding far back. Also seen in this picture: the inability to see the above corner.
In my opinion, the windows peeking into last
need to be two-way. One way windows only exacerbate the holding problem: teams holding last will have an impossible time anticipating any sort of movement by the enemy team until they're already far into upper. Plus, the defenders can't approach the windows at all to spam because they have no visibility while the attackers have full visibility. The attackers can see every move defenders make while the defenders see none of it. Think of someone watching you while you sleep, that's what this feels like.
Peeking in this far is a death sentence because defense can see you from a mile away while you can't see them at all until you're up in their face:
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And that's it. Lots and lots of feedback. If you have any sort of questions about stuff I covered here feel free to message me or respond, but I think I covered it all pretty well and explained my thought process.
5cp is fun.