I ditch the battlemechs, and focus on having a strong space navy with lots of ASF's.

For defense, a garrison of ASF's is enough to ward off any invasions from dropships without a strong escort.

For offensive operations, our warships can just orbitally bombard any troop formations that aren't close to urban areas, then we use VTOL troop transports to drop off recon battlearmor infantry, and they use TAG to designate targets for stand-off Arrow IV missiles launched from ASF's.

By the time we have to actually engage in house-to-house fighting, the enemy has been reduced to just their infantry, which our own, superior, battle-armored infantry can clear out one building at a time.