https://github.com/privacyguides/privacyguides.org/blob/HEAD/docs/email.md and https://www.privacytools.io/privacy-email document services you’ll probably find interesting.
It’s probably about as likely that other email service providers will shut down (or at least make the service they provide worse) unexpectedly.
A more sustainable solution would be to make it easier to self-host similar services and giving your contacts more help with transitioning to more privacy-respecting solutions.
I’m personally not very worried about proton.me or tuta.com shutting down without giving me much time to replace them, but it’s a possibility I keep in mind as it would be inconvenient if that happened, and I can do a little bit to make it less inconvenient if it does happen (e.g. by registering alternate email addresses with any person or company who wants to talk to me).
Why would you need an app when an account settings page and a web interface for posts exist? What need is that combination not meeting for you?
Please share what you’ve learned by suggesting changes to https://github.com/privacyguides/privacyguides.org/blob/HEAD/docs/desktop.md