We have actions for moving Main or Considering to Side, Side to Considering but nothing for Moving things to Maindeck anywhere. Feels a bit like an unfinished card cycle at this point!
My suggestion would be ‘Alt+Spacebar.’ Not all key boards have a leading ‘~’ on their number row and the numerical space is already fully utilised. “Alt+Space” is very compact, present on all keyboards and can be done with just the thumb when we’re feeling lazy enough to be efficent.
Vote for Alt+Sp

The same keyboard shortcut that moves to those boards also works to move them back to the main deck. The UI does need to be updated to reflect that though.