It sounds to me like an active heavy bodied beginner snake would be perfect for you. The problem is that I can't think of any that check off all those boxes. There are active beginner snakes; there are heavy bodied beginner snakes; and there are active heavy bodied snakes for intermediate keepers, but I can't think of a species that is all three. I think you are going to have to compromise on your criteria a little bit.
I'd advise you not to compromise on the "for beginners" criteria, however. If you've never kept reptiles before, their care can be counterintuitive. With mammals you can rely on your instincts a little bit, but not with reptiles. Their psychology and biology are just too different. So, you'll want the type of docile and hardy snakes that are forgiving of the mistakes that inexperienced keepers make. Most lists of the best beginner snakes include: corn snakes, ball pythons, milk/king snakes, hognose snakes, and garter snakes. Boas don't usually make those lists due to their size, but they grow slowly, so you'll have time to get used to it before it gets big.