Looks like you don't really want to sit for ABRSM exams, and problaby not necessary. You'll benefit most from the SOFT online learning. There's one problem - interest level not high enough to start one on theory.
By the way, if you think you have about Grade 5, you should be OK. If you are thinking of composition, then you'll have to go above that.
By the way, you have not entirely answered sof's question. What benefit do you see theory for yourself? What do you think theory can help you with?
I'm not asking this because I'm against theory. In fact, I'm an ardent avocate for theory. However, knowing what we want to do with it will make theory "practical". Otherwise, head knowledge can get you nowhere.