But you can always go back and rest, so spending every per-rest ability in a fight is what defines your real power.
If you say: I don't want to rest-spam, you are asking: who is the strongest with one arm tight to their back, or any kind of handicap, which is all just a matter of taste.
The real challenge of the game is only in the very hard boss fights. All the other fights are just a balance of how much resources you want to throw at it, but you have to make that decision yourself. The game itself allows you to make those smaller fights as easy as you want.
In my first party on Normal difficulty I just did the first White March fights, but I have now levelled up so much, that it has not been much of a challenge (except when I prematurely thought I was going to lift that siege near my castle )
So in the mean time, I have started several runs on hard, discovering new classes, and by now I have won my castles for the fourth time, and I am starting to regard Chapter I as a set of know puzzles that I am solving ever more efficiently.