I went through my stored logins to migrate the vast majority of my accounts one by one (and deleted quite a few old and forgotten ones in the process). Took a couple of hours, but went mostly well.
For everything that I might have missed, I have gmail set up to forward everything to my new address. The new address (I went with posteo myself) has a filter that automatically moves stuff addressed to gmail to a separate folder. Whenever something ends up in there, I go and migrate or delete the account.
You can probably (never used proton) set up a filter on the new address to mark or move stuff that was originally sent to gmail, too. Helps visualize the accounts you need to migrate/update.