Would highly recommend asking on the Anbennar discord and/or the subreddit. Tends to be more responsive for such questions.

you can take greenskin capitals only if they have no tribal land anymore - adventurers u can only vassalize or force to migrate until u are settled - once u reach admin 7 u can form a regular country - u should complete the generic mission tree for adventurers and all reforms though to avoid the debuff

and yeah I second checking out the discord or r/anbennar - probably more info there

also get a federation asap and get federation wars

I had to check where Esthil was, because it is not in the game at the beginning. But i assume, OP meant one of the smaller adventuring countries in castanor, which then later become Esthil?
Just test them out, play some time and see how it goes.
They are a lot like trying out some native american nation in the basic game. You have no clue at the beginning but might find it interesting.
The adventuring countries first have to settle down, form a coalition to then get strong enough to form a decent nation.
At the beginning you cannot do much outside of attacking your neighbours for loot and prestige. But after that you become stronger quite fast.
And you have to wait for the right tech skill. But that counts for every nation.

Edit: i finally found it in the decisions and policies: to form Esthil you have to form your own country. Which you can do with adm tech 7.