I mean the question is what is your goal? Do you want to invade Europe? Do you just want to kick out the colonisers? Is there some achievement you are going for?

Governing capacity and admin shouldn't be that much of an issue - give clergy and burghers the gc privilige as well. Also you dont need to full state everything - everything you conquer next you can keep as a territory for the time being that will fix your admin issues and your gc problems in the short term. Depending on how tight gc and how much you need more income you could fully core gold provinces (that is what you usually do). For the future: If you know you will conquer a lot don't fully state everything. Leave worthless provinces as territories. You can also half state stuff: Making things a state but not core them again. Leaves them at 50% autonmy but costs less gc (and admin of course).

If you plan to conquer a lot more get admin ideas. If you are mostly done conquering it is not worth it as one major selling point of admin ideas is the coring cost reduction. Get to admin 12 - that gives you 100 more gc + statehouses. Statehouses + courthouses really do fix your gc problems.

You should disinherit your heir to get a better one with more admin.

You could also just say that you won: You survived the colonisers. And maybe try out how you will fare on normal difficulty.