It looks like the bb are taking care of some of the ammonia. That's good. I wish I could tell you more about this, but I don't know much about it. I think you can put hydrogen peroxide in the tank to kill some snails, but you should do some research on that, as I don't know much about it. And, assassin snails eat snails, so some of those might help.
Try plucking the snails out by hand. When buying plants, dip them in a hydrogen peroxide dip to kill any snails/eggs. I have had a problem with Malaysian Trumpet Snails in the past. I didn't treat plants that I bought and I got the mts. Bought some assassin snails and they are keeping the mts in check. I'm not sure if they will go after pond snails though.
I have some assassins in my 20 they go after the pond snails first. Before the mts and ramshorns.
Update:
Joe and Marley shrimp are still alive! Not only that, they are thriving! They are eating like piggies and growing. They've molted once and I think they're due to molt again.
I haven't done anything about the snails yet. They're still small and pretty cute actually.
Exciting adventure you have my friend, i would like to ask, what do you mean by molted?
(not that i know anything about shrimps etc just curious )
What do you mean by molted?
Sounds very interesting
I will do some research on it and maybe and 1 or something ...