Hey everyone, thanks for the info!

The frog has no ridges, you are correct. It does look rather like a bullfrog, except that most photos I see of bullfrogs they have green heads. This one is brown all the way over. It also seems like this one has a more pointed snout than a bullfrog. If they are bullfrogs then that explains why they're taking so long to change, I guess.

I thought I knew what a bullfrog tadpole looked like, but the guide you linked says that "Rana catesbeiana tadpoles are extremely variable" and has a photo that looks very similar to leopard frog tadpoles to me. Just from looking at tadpole pictures I don't feel confident saying its a leopard frog or bullfrog, though I agree that it looks nothing like a river frog. I ASSUME it's a North American frog but who knows?
a couple more photos of mine: