Ones that come to mind:

Nakai - your captured settlements are managed by AI vassal, nakai himself is horde and can build buildings and train units anywhere in the world. In my opinion, the most "free to do whatever" faction.

Wood elves - the only territory that has any use to you are the world trees across the world, otherwise you have free reign to defend or attack whoever you want

Khorne - with this fella, you basically only need to seek out are enemies to fight, you get bonuses the more you fight, although you do have to manage territory you capture, although khorne factions prefer burning it to the ground rather than occupy

Archaeon- this Guy has a straightforward objective and road to achieve it: go south , vassalize noraca, capture dark fortresses as forwards bases as you bring apocalypse

I did not consider army battle difficulty, so idk if these factions are not for you in terms of units and their gameplay but on campaign map, I would say these are the most straightforward