Hmm. Not letting pawns in is not the best way of getting them to pay.
With 20 games at a time you could still, in theory, play hundreds of moves. Less likely I guess. I would change from 20 games to 100 moves (it seems to me that i play a lot, but rarely get to 100).

By the way I also prefer this site to iyt for many reasons, which is why I'm trying to think of ways that it could be accessible more often.