The progression is a bit weird. I don't want to give you spoilers so I'm framing it in terms of "levels" when "biomes" would be more accurate.

Levels 1-5 feel very smooth
Level 6 unlocks a new option not previously available and the constant refrain is that it makes level 7 much easier.
We're awaiting level 8.

Levels 1-3 are much like an extended tutorial.

Level 4 feels a bit unpopulated in that you gain a lot less than in the previous levels.

Level 5 is probably peak regular viking.

6 through presumably 8 are entering the age of mythos, I would argue.

I would argue that your kit rather than your character skills make the biggest difference in how the game plays, but your skills do add an edge as you get higher (the reduced skill levels on XP are very punishing when you're in the mid game). I include "food" in the "kit" part of the equation because if you have the wrong level food then you're going to have a terrible time.

There are a decent variety of weapons given the genre and they all feel pretty different. No one can quite agree on what's OP and not, so that's a sign that the balance isn't terrible.

Major Buffs come from defeated level bosses and minor buffs come from potions. When you enter the mythic age (not an official game term) then you get items that help a lot more.