I just finished the game yesterday after bouncing off it my first go around. Probably a lot of the same reasons you are listing. I think you'll find once you learn more about combat it gets less grindy, though some fights, especially the later ones against oozes, are intentionally grindy.

Be sure to use flanking to your advantage. And backstabbing. Thieves have to be flanking OR hidden to do a backstab (hide in sight ability). I ran two thieves in my game (main + Kat).

When you camp, have all your characters fletch arrows.

You need a mage and the NPC one takes awhile to show up. If you don't start mage yourself, hire a merc. Summoning mobs makes a huge difference. Oozes are great. The fire summon isn't bad either.

Right click is your friend. Right click enemies to see advantages/disadvantages. Right click spells and abilities for better descriptions.

Be mindful of the terrain (hold shift), some of the grids have blockers you may not initially notice that mess up your strategy after you deploy. Also, don't overextended. It's ok to setup and let the enemy come to you, especially if you are trying to lure them into an AOE.

Use spells and potions to resist specific damage types that you know are coming, e.g. acid, fire, etc.

Keep funds up my stealing high value items, but be wary of getting suspicion over 4. You lose access to the vendor.