Most of the mobs, other than looter/bandit/highwayman, have some top tier units among them numbering around 1 to 20(At least in my playthrough) according to their overall unit number. Heretics are pretty strong in that regard as well, heretic invoker and heretic magnus' are pretty solid units that you can only take out with couched lance damage at the very early game(since you can't get a decent sword and armor)

Instead of trying to kill heretics or snake cult or any resemnbling mobs, I suggest getting some decent money from trading or from arena. Arena has a pretty bountiful reward of 500 denar for killing 20 men and 2000 denar for being the last one standing( It might be hard if you play on higher difficulty, but still worth your time) Trading, if your skill is high enough(With right choices you can get trading and invenory management to 7) is actually better. Fierdsvain to sarleon, then from sarleon to D'Shar has some profitable goods you can trade. Flux bundels from Valonbray gets from 70-90 and sells at 200+ in Avendor. Wool is also around a hundred and you can sell it around 200, but I don't remember whether if you get it from D'shar or Sarleon and sell the other.

Other than that, gathering troops is a tedious task, improving much more difficult. Your best shot is to stumble upon eiher adventurers or jatu outcasts(That's my personal choice.) They cost hella high but unlike other units give their worth in battles. A few adventurers and a dozen soldiers from the local culture you want can make it both safer to travel(Fast-travel is a no no in the mod, if you haven't figured out already) and can help you take out some bandits first to gain money.

Most of the early game, questing and grinding helps you from my viewpoint. After a while, once you have a decent army around 40-50 fine soldiers, hunting Jatu or Mystmountain is helpful as they have good loot and a chance to drop various supplies, which is pretty expensive. After that comes probably the mid-game, and I don't want to spoil much of it.

Beware, I just told what I most of the time do at the start of my playthroughs to gain a decent start so it might not be suitable to your tastes. But I think it might be helpful.

Cheers!

The merchant quest is no longer a "tutorial/starting quest". It is just a "quest", so you have to prepare yourself first.
And the beginning if given party is displayed on the map as "heavy cavalry/infantry" icon, then it is better to avoid it. Focus on "poor looking hunters" groups (NOT hunters wearing white hoods).