6000 AIs minimum? Good luck doing that in any engine. I hope you know what you're doing and you're not just some newb kid thinking, "I'm going to make a game like X only 10 times as big! It'll be awesome!" I'm sure it's doable, since the units probably won't need to "think" very often, but you'll probably want to use C++ and need to spend a bunch of time benchmarking and optimizing it.
Speaking of benchmarks, I've been checking out Bunnymark. It might be interesting to you. That version is by far the fastest I've seen. I get at least 10,000 bunnies before it starts to slow down at all (below 60fps). Someone also made a Godot version. I got 4500 bunnies with that at 60fps (with a build, in the editor it is much worse). I also tried a version with Atomic engine, which uses javascript, I got about 5300 bunnies with that. GDScript seems pretty slow, though from the bunnymark I guess it's not as slow as I'd feared.