Why make C-a your prefix key? Just because it's the default in 'screen'? I use C-a a lot to go to BOL, just like C-e to go to EOL. I also use C-b a lot with C-f.

The obvious choice is C-z.

You rarely suspend processes and when you do, C-zC-z is pretty quick. For other combos, it pays to have control and z close to each other so that when your little finger and ring finger operate C-z then your other fingers can freely type whatever follows the prefix without having to unwind your fingers back from the keyboard first.