Have you tried using the series finder with yaoi/shounen ai and mecha selected? There are 25 different results from both searches just incase you haven't read tried that yet.
The omegacerse has mech. If you want to see one in anime i believe gundam iron blood or orphan has a bl side couple
Lots of the quick transmigration novels have mecha arcs What to Do When I Become a Koi and Fall into the Male God’s Bathtub Yin Yang Eye GunGun’s Marriage Contract Warhead Pet Hamster