For Sci-FI 6-12mm I'd offer the following; I recommend them all to try; each is a quality game, with their own "quirks" that you'd have to try to suit your taste:
Polyversal-Collins Epic Wargames. You basically build your force from the ground up using their free Polyversal Online Arsenal after you purchase the game. Easy to use, stable platform. Allows you to print their special polygon unit cards. Lots of weapons and traits to choose from. Good for combined arms, and you can throw in your Battletech mechs. Neat command system, fairly "hard" sci-fi, but does feel like a WW2 game sometimes. Combat resolution rolls take some getting used to..Fits your battalion level requirement quite well and probably the closest to what you are looking for; 40 bases would not be a problem with this system.
Battle Suit Alpha-Wiley Games. Not a BN level game/say platoon/company; really only 4-8 individual units per side is best. Really more of a Battletech light game, but allows you to build a combined arms force. Mechs are the star of the show, but plenty of rules for other ground and air forces. Easy system to learn and use, like above, build your own force. Fun, not quite beer and pretzels but close enough. Can play easily 4-6 players, each with their own unique force.
Horizon Wars Midnight Dark (updated from the original rules)-Precinct Omega. Again, not quite BN level, but 10-15 units max (elements could be stand ins for platoons). Little more complex than BS Alpha, but once you get the hang of it, pretty fun. Good combined arms, build your own force; again Mechs are the most customizable…but a small mech force can easily be beat by a larger combined arms ground/air team.
Steel Rift, Death Ray Designs. Again, a mech centric game, but you can build light vehicle squadrons into the game system, again, highly customizable. Probably farthest to what you are looking for, but I recommend it nonetheless. Their new line of customizable mechs and vehicles are HIGHLY recommended; some of the best mech/vehicle designs currently on the market IMO.
Of course, Battletech Alpha Strike if you want to use your vehicles as stand-ins for in-universe vehicles. I think it is a great game, but not really a BN level game.