My son (who also posts on this board) will tell you that I was a Baker Mayfield fan when he entered the draft. Did I think he'd be a Superstar? No. But he had good skills, was a good leader, and a whole lot of moxie. He can be a Superbowl winning QB.
I see that in Bo Nix. He'll be downgraded because of his age, and because he doesn't have the dazzling physical attributes of the bigger named QB candidates.
That said, I think he is a starting NFL QB with plenty of size and physical attributes, and will be a good one. He will, of course, have to be in a good stable system that he has a minimum of 1 year (ideally 2) to be coached up and learn the system. He will need to have a good team with good weapons around him. It would be terrible for him to be drafted and expected to start anytime during his rookie year, or by a team that is immediate QB needy and isn't very good.
The Rams would be an ideal home for him. But, I can't imagine the Rams making him their #19 pick. Maybe the the final 1 or 2 picks of the 1st round in a trade down that got them another 2nd round pick, but I think the Rams would have more interest in him in the 2nd round....although I still don't think they will do it.
As an aside, I think the Rams might be looking at a prospect like Jordan Travis in the 3rd or 4h rounds. This kid is oozing with potential and could be worth the 3rd round comp pick if they're having some luck getting the guys they have circled using their first 3 picks.