See this is why I suggested significantly amping up things like damage and damage bonuses, wars should change significantly every single turn, attacking over a river against a unit in the mountains should be tantamount to 1 turn suicide. Attacking a unit flanked on 2 sides should see an incredibly easy 1 turn victory.
That combat has become such a grind that this sounds reasonable is a reason to change it so that each move actually feels like it matters and is the difference between success and defeat. No amount of doomstacking is going to fix this and will in fact just make this worse, look now your stacks have tons more health, whittling away at it now takes infinite turns!