Giving Hey a try, to see what all the fuss is about now there's no invite necessary. I've started forwarding two of my legacy service accounts to it, as they should be a reasonable test. I can't forward my main Proton Mail account, because they don't implement forwarding, but that's fine for now.
It'll have to really blow me away to get me to subscribe and fully switch. Pretty much all of my personal email is transactional these days – receipts, newsletters, etc. I can't remember the last time I wrote an email to someone I actually know… messaging apps seem to have filled that slot instead.