Imac 2020 keyboard not holding charge

Switch keyboard off when not in use, preferably with the charging cable connected. If that’s not satisfactory, you can get a free repair estimate by scheduling bring-in repair https://getsupport.apple.com/. It’s probably easier/same cost to buy new. All the best!

Using the charging cable connected to the Mac directly effectively makes it a wired KB.

I agree with hcsitas, repair cost will go a long way toward buying a new KB if you have to have wireless.