Disclaimer: I actually have no idea if this would work, it’s just a theory. Since you have a test project set up, perhaps you could see if it works.
How about creating a dummy game object that references all the tilemaps you use in the game. Spawn this in your main menu collection using a factory set to load dynamically, but just hide the game object away (out of z range, for example). I’d load the factory after a small delay, so that the game doesn’t take a long time to launch.
I would think this should create a reference, which would then mean the tilemaps would not need to be loaded again when switching collections. Am I right, or am I fundamentally misunderstanding something here?
The drawback of course is that your game’s memory usage would go up.