I would just make the Titan vulnerable to overpowering numbers, tbh.
Make it powerful against capitals, in fact, make it horrendously powerful against them. Able to scare caps off just by showing up. But make it fall to an actual army.
Ofc, the balance would need to be so that if I invested 60%+ of my pop into caps, then I cannot beat a titan with the remaining army. However if I got only 1-2 caps, supported by a large army, I can wear the titan down without taking anything equal to it's cost in damages.
HOW to do this, hrm... maybe a reduction to shield / armor if more than X non-strikecraft are attacking a Titan, causing large armies to take it out exponentially faster.