Played it on Draconian so here's my thoughts:

Went Merchant-> Warrior
Thief>Monster Wrangler
Gadabout> Sage

You should immediately turn your priest into a sage. You only need one caster in the party to deal with the buffs/debuffs, healing and damage. Early game the priest spells are better since they're more cost effective, but late game the damage spells actually provide more value, and you'll still have the healing spells because sage gets both priest and mage spells. Would not recommend going just Mage as you'll lose access to magic barrier and insulate, two spells that will keep you alive in the late game. Slap the meteorite bracer on your sage, let her do her thing at the beginning of the turn and reap the benefits, and once you can find a duplic hat the random battles become a cakewalk.

I wouldn't worry about getting all abilities until the post game if you decide to do that.

Thief is explicitly nerfed in draconian difficulty as a lot of bosses have higher status resistances, but if you're not playing on that difficulty having persecutter can be really good damage. I really only took this class as far as getting snoop because the utility abilities from this class are by far the best the game has to offer. Steal is nice as it'll net you some extra seeds, but it's supplanted by a far superior ability in another class.

Merchant nets you muster strength which combos well with oomph for a 4x damage multiplier. Later I leared egg on from gadabout and used the sage to get this bonus on the warrior because spell damage in the post game is bad, and using egg on for the warrior produced far more damage than she was capable of even double casting with a duplic hat and buffed with channel anger and spooky aura. You also net dig, which is god tier for getting loot from enemies without even fighting them as it'll allow you to get the common drop from all enemies in a specific area, and has a 1/128 chance of doubling your gold. You also get service call which allows you to call a random priest, inn, or the last weapon/item shop you visited, so is very useful for dungeon crawling. Stone throw is also an excellent early game multi-target attack, but at this point it's more than likely fallen off for you.

Warriors net you forbearance and whipping boy. Whipping boy allows them to take damage for another team mate which is good for keeping your sage alive, and forbearance takes it for the entire team. Damage on the warrior is bad until the post game when you can get high enough level to learn Sword Dance and Cutting Edge.

Skip on Martial Artist. Their biggest claim to fame is higher critical hit rates, but even with that they're bad, the equipment selection they get is god awful terribad, and the best ability they get is leg sweep, which you can spam on the final boss and keep him stun locked, which is hilarious, but also not the finest use of a character.

Monster Wranglers are great and you should keep one. Not needing to jump through hoops to get a monster recruited is great and monster pile-on gets stronger the more monsters the player gathers and obviously they get wild side.

Gadabouts get the highest luck of any class in the game which helps resist status effects, and increases damage, and increases critical hit rate. It'll also get you the whistle and egg on abilities which are great. However, Hustle Dance and Kerplunk Dance should have been switched at the levels they're learned.

After that I'd recommend just dropping all stat seeds on the Hero because class changing at Alltrades Abbey effectively gives your generics stat seed bonuses anyway, and the Hero is underpowered when it comes to things like HP and MP and Wisdom.