I migrated from gmail to proton then to fastmail and I can now really appreciate how much more polished gmail was.
My biggest pain is fastmail's ios app, e.g. clicking on an email notification will in 50% of cases result in fastmail's app to open in an unresponsive state where I have to forcefully quit the app and start again. It's been like that for close to two years now.
I think most business people dont care about clients at all because they use IMAP/SMTP/carddav/caldav.
Once you have more than one mailbox then its kinda the only way. So apple mail, thunderbird etc. And then you mostly get some quality provider mailbox.org, fastmail, infomaniak or whatever but its all the same.
