Kirk Cousins, Jimmy G, Daniel Jones, Jake Browning, Marcus Mariota, Andy Dalton, Jacoby Brisset, Gardner Minshew, Cooper Rush, Zach Wilson, etc. Russell Wilson at this point (even though he is technically starting, but he is basically a backup level QB now). Tyrod Taylor is still maybe above average as a backup.
I was actually really hoping the Bills would sign Zach Wilson as a backup. He is athletic and has a big arm. He is a good kid and would probably really benefit from being with a strong organization and Josh Allen. I think he would be a good backup.