Why not just use proton?

A few of the many reasons not to use Proton:

• their e2ee is snakeoil (see my comment here about why - but tldr it requires completely trusting them and if you completely trust them you wouldn't need e2ee, the point of e2ee is to avoid needing to trust the service provider)
• their server-side code is closed-source
• they're a freemium service which can and does arbitrarily decide to start charging for previously-free features
• they've suspended a number of users who they should not have
• their CEO is a trump fanboy.

...

Its Swiss based.

You know who else was Swiss based? 🙄

Not sure about purism but I think its US so avoid it like a plague.

I don't know enough about Purism to endorse them but afaict they don't have any of the above problems.

Purism's e2ee is PGP; you can use their service via their client software or whatever other client you want, and can communicate with people who are using different implementations with different mail providers. I don't see any mention of them even offering webmail but I expect that if they do they would probably offer PGP there using a browser extension instead of having extremely-impractical-to-verify-before-running-it js code being sent anew from the server every time you load the page (which is how Proton's webmail works, and also what they offer for non-Proton users to receive mail encrypted using their nonstandard encryption).

I'd rather have US legal jurisdiction and credible e2ee which doesn't allow the operator to trivially circumvent it for targeted users than to have Swiss jurisdiction and snake oil.