Or do I have to get a full provisional license? (As in not just one as a stepping stone for the test)
As said, you have to pass the UK driving tests if you want to continue to drive as you only have 12 months grace on your US license from when you first resided in the UK. That 'up to 12 months', is from when you arrived and does not reset to zero every time you leave the UK and come back again (thats been tested in court).
To take the UK tests you will need to apply for a Provisional Licence. When you get that PL, you can book some driving lessons and the instructor will be able to tell you if he thinks you will pass or whether you will need more lessons to pass. Don't underestimate how hard the UK driving test is.
If you haven't passed the UK tests within that 12 months that you arrived in the UK:-
-You will then be regarded in the same as any other learner driver in the UK and have the same restrictions
https://www.gov.uk/driving-lessons-learning-to-drive-You must stop driving, because to continue on your US licence will be a criminal offence (you go to court) and the points you will also receive will be held on a ghost licence, to be put on your UK licence when you pass that. New drivers only get 6 points before their licence is revoked. Plus your insurance would be invalid (another criminal offence) and your insurers would not pay out on any claim.
Basically, if your want to continue driving on your own, you need to pass the UK driving tests before your 'up to 12 months' has ended.