If you really need to talk to someone over email securely, PGP over email is the way to go.
I've never had the need to protect my mail (using PGP for example) as I don't send mails that's sensitive enough for it or are forced to message with tech illiterate people (sorry) that can't use it.
I just want a privacy friendly service that won't give up my data to uninvolved 3rd parties :(
A lot of it comes down to reading over a company's privacy policy (if you can't understand it / its really long, that's a red flag), where they are located, and -- important -- if its free or paid.
Here's the write-up that convinced me to switch: https://drewdevault.com/2020/06/19/Mail-service-provider-rec...
Protonmail is mentioned in the footnotes.