As long as you never send any emails to anyone who uses Gmail or Yahoo or Outlook etc.

Both Thunderbird and Outlook have plugins for PGP and S/MIME. You can use them to read/send mail from Gmail and Yahoo domains as well.

Edit:

If you're using them for an organization that uses Exchange/O365, the admin would need to make sure IMAP/POP protocols are enabled over TLS/STARTTLS, as opposed to using EAS. However, if you're dealing with mail in an organization, you're probably not using your own encryption keys to being with.

See https://thehelm.com/

What are the marginal benefits (and costs) of running a Helm over ProtonMail service? I can see none.

The justified concerns the security community has with ProtonMail is: Crypto in the browser is bad (mitigable with Qubes), and How do I know PM isn't serving me a backdoored JS.

IIRC, Helm has auto updated binaries so backdoor-free code isn't a delta. The best I can come up with is: server+CPU observation/isolation is stronger on local hardware relative to PM at the cost of network observation. Hypothetical: "Ok a Spectre-style attack is out, spam emails and let's do some timing-correlation traffic analysis"

I actually own a Helm, and I like it very much!