Since you already get some rights from the gmail oauth, you might as well just add more features for users, like the ability to reply to emails from gmail and have gmail messages automatically deleted (since they are being sent to proton).

You could make them extra features that can be turned on with a pop-up from Google asking for permission.

This will make it easier to move away, but you can still reply to Gmail if you need to.

and allowing users to auto delete the emails is just a nice feature. It would be better to put them in a queue to be deleted from the big Gs computers than to let them stay there forever.