Radius culling is basically just a laggy and inferior version of the cordon tool. In cases where you need to only compile a specific section of your map at a time, cordon tool is always the way to go.
I also took a look at your video - the first thing I'll say is that the map is far too big and open. I assume you'll get around to building a more interesting layout since it's pre-alpha, but starting with a giant empty box is generally considered a bad mapping habit. The skyscraper is (supposedly) your centerpiece, so start building the map around that, and then wall off the play area when you're done. Keep a few player model props sprinkled throughout for a size reference and you should be good to go.
Second, the design of the skyscraper itself seems like it will be funneling everyone in through one entrance. You're going to want a few side and/or back doors to give players options when approaching the point and prevent the holding team from locking down a single entrance. Also consider how you traveled around in your map using sticky jumps - most classes don't have that luxury and will be significantly slower. Don't make them run all the way around the building just to get inside!
Also, to prevent players from jumping on top of the spawns, just add a brush with the playerclip texture
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clip: that extends from the roof to your skybox.