Hmm, the general theme progression for a map is Neutral > Facade > Industrial > Spytech, or something similar.
I think you could try Outside (if applicable) > Cave > Used Cave > Spytech.
The "Used cave" section is a large area inside your cave that is an active workplace, maybe it's being mined and there some sheds and walkways built, maybe their building a large tunnel underground, anything that is a legitimate and fitting industry.
You can use this industrial build up to transition to the spytech areas. E.g A build that is just an entrance, a bunker door, a large gate for trucks hanging open, a small cave that runs under the facility. Try to be creative but don't get hung up on trying to find the perfect solution, just think on it a day or two and use your best ideas.