I've cycled through several email clients through the years and have more recently settled on Outlook (gasp) mostly because all my clients and 'employers' seem to standardize on that.

I've been using ProtonMail across platforms (without need for web based UI) probably since they became a thing with absolutely no issues. But then of course those emails become local to that particular machine and are not synced across devices unless a redundant copy is left on the Proton IMAP server. If I want certain emails saved locally I just archive them to a local folder. Most of my emails (inbox, sent, custom folders, etc) are maintained on Proton servers so I don't need to fuss with VPN connections, dynamic DNS, or any always-on systems. I haven't tried ProtonMail service with Elm or Pine but it does work perfectly well with Thunderbird, Outlook, and likely any client that can point at an IMAP/SMTP provider. That port is only exposed to the respective desktop device, so no need for anything complicated. On desktop devices I run and just point it at the localhost port that the bridge process opens. On mobile devices (mostly iOS) I use the native apps that Proton provides.

Protonmail bridge mac