Having said that, I've only ever played a max-game style which gives my party greatest chance of success (ability to GM useful magics & best weapons) - not one of those parties I like aesthetically but which will be an extra challenge.

I'd encourage you to continue - the newly added Act 5 is an interesting play, but there's still some "hack & slash with little refuel possibility" in your future after the ToaTT before you get there. If it helps, I pretty much settled into a pattern where the main job of my 2 spellcasters became to keep the buffs & protections up to date while my tanks (slowly) hacked away. Many of the buffs have durations measured in a few turns & by the time you've got the last one set, it's pretty much time to re-cast the first one. There is definitely a significant increase in difficulty if you try to fight "naked". It does mean that the spellcasters spend most of the battle keeping you alive rather than "attacking", but it works.

The crucial buffs are (from memory) Liquid Membrane, Celestial Armour, Stone Skin and the resistances (including Arcane Ward). You'll find that Celestial Armour gets chewed up almost with the next attack in the later game so you may find yourself casting every turn unless there's something else to cast. The stat buffs like Eagle Eye are less important after about mid-game. Liberal use of Sleep to keep the opponents at bay as well as Poison & Pain Reflection also help. And Burning Determination is something you have to always keep active if you're facing enemies with the paralyze/knock out ability.

And potions. LOTS of potions. This isn't like MM6-9 where you eventually get to a point where you virtually never need them - pretty much the only time my spellcasters weren't re-casting a buff when when they had to top up mana - which was every 5th or 6th turn. Ditto health - maybe every 10th turn for each character was a potion