Google have confirmed they will provide limited migration options to a 'no-cost option' (likely a Gmail account) sometime before May.
Upgrade from G Suite legacy free edition - Google Workspace Admin Help
I'm hoping they will allow you to merge with an existing Gmail account - but if not, it may create a 'zombie' Google account with your purchases etc stored on it for reference.
Otherwise so long as your domain is managed separately from Google, you can just point it to individual Gmail accounts (or Outlook or whatever).
This won't be a pleasant migration, which is why Google are hoping you'll just pay up.