Sender and recipient can't be encrypted e2e. How would the server know to whom deliver the email if those are encrypted and not visible to it?
AFAIK tuta encryption extends to the subject line only.
Still a nice addition, don't get me wrong, but I believe you misunderstood something.
From their own doc:
The only unencrypted data are mail addresses of users as well as senders and recipients of emails.
Contacts and everything else is encrypted similarly in all "secure email" providers, including Proton.
Security is hardly a binary property.
Given you mention the specific technical setup, I would say yes - that is secure against most risks relevant for most people.
At least, it's totally fine according to my own threat model, where I looked specifically at broswer-based encryption vs "manual" encryption (I.e. using PGP tools locally).