For certain incoming emails (rule "if from = ..."), offer the option to notify another email. Just "you have email in proton". Not forward the whole email as auto-forward does.

I have two PM accounts, and I always get such brief notification in my primary account when there's new mail in my secondary account. I've looked and can't find where I set that up.

I occasionally send personal messages to family that I'd rather not share with the googleverse. One person still uses gmail, and I've suggested that they get a free PM account.

If they had a free PM acct with the ability to notify their gmail, (1) I could send to them with confidence, and (2) they could see what PM is like.