OMG. So I've been playing a self-imposed hard mode.
So for anyone as retarded as me - after the battle there's a small text in the bottom left corner saying "Hold H to Heal All". And I noticed it only 13 hours in.
Yes, you can instantly replenish all your slain troops after the battle for a small sum, as long as at least one member of a stack survives, instead of running back to the recruiter and buying units. So if you lose half of your army in a battle it'll costs you a few hundred instead of thousands.
I must've lost like 30k gold in the course of my playthrough for buying new troops instead of healing. And the whole time I was thinking "holy shit this economy is tight, very punishing for new players and casuals".
So yes, I should pay more attention but it's a retarded mechanics nonetheless. Resurrecting troops was an entire complex feature ecosystem in the old games, with units, heroes, NPCs and spells doing the task. Here you just press H.
The game is relatively entertaining but god, it's such a decline compared to the prequels.