Proton kept popping up massively recommended while some occasional critical mentions from folks in anarchist circles, etc - made me a bit 🤨 and want to dig in more,

No surprise that folks in anarchist circles are skeptical of Proton ha. That said, I do know quite a few people in the email "industry" who are broadly skeptical of Proton's general philosophy/approach to email security, and the way they market their service/offerings.

Others I poked into are fastmail and tuta - both seem a fair bit better. Might be worth a look

Fastmail has a great interface and user experience imo, significantly better than any other web client I've tried. That said, they're not end-to-end encrypted, so they're not really trying to fill the same niche as Proton/Tuta.

From their website:

Fastmail customers looking for end-to-end encryption can use PGP or s/mime in many popular 3rd party apps. We don’t offer end-to-end encryption in our own apps, as we don’t believe it provides a meaningful increase in security for most users...

If you don’t trust the server, you can’t trust it to load uncompromised code, so you should be using a third party app to do end-to-end encryption, which we fully support. And if you really need end-to-end encryption, we highly recommend you don’t use email at all and use Signal, which was designed for this kind of use case.

I honestly don't know enough to separate the wheat from the chaff here (I can barely write functional python scripts lol - so please chime in if I'm completely off base), but this comes across to me as an understandable (and fairly honest) compromise, that is probably adequate for some threat models?

Last time I used Tuta the user experience was pretty clunky, but afaik it is E2EE, so it's probably a better direct alternative to Proton.