I am a paying customer for ProtonMail, Fastmail, and Google Workplaces (renamed GSuite). I currently use Workplaces as my daily driver but keep evaluating the alternatives.

One thing that ProtonMail really needs is automatic detection of calendar events in emails and creation of calendar events. It is a real blocker to have to manually create calendar entries. When they release a new iOS app that supports their calendar, if they support automatic calendar entries then I will likely switch back.

The other big win with Workplaces is Cloud Search, which is also a big time saver, but I could reluctantly live without that.