[Aerich]

I would miss early access to Static Charge, Spike Growth, and Entangle.

AFAIK, the equation I showed is only used for the start. The equation you showed is the one used for subsequent level ups, except the 0.5 is rounded down.

If you don't want a bard or a pally but want more power, add a pure fighter. IWD is more battle-heavy than BG, and having two or three dedicated tanks on normal level is not overkill. Every map swarms with enemies. Your R/C, F/M, F/T combination lacks a pure grunt - the R/C should not use a shield because of the way two-hand fighting is implemented, the F/M won't be durable until it gets stoneskin, and the F/T won't have the spell buffs.

A druid would also go well, as the R/C and F/M won't get access to any summons worth having (other than undead) for a while. Mages really don't get any good summons until spell level 5 - goblin elites for a lvl 4 spell?? Another mage wouldn't be bad either, as most good spells have two copies.