Since encryption (and decryption) is done by client-side Javascript, and you don't want them to run Javascript, how would you prefer the encryption be done?

I didn't realize they were doing an end to end or zero knowledge system.

okay, then I'll do that in whonix.

regarding the payment I couldn't see how to pay in crypto and also get a simple captcha instead of SMS verification

This is false.

I've paid Protonmail using bitcoin and that is all that was needed.

how?

I just did the $4 plan right now, and it is asking for credit card, paypal. I do see the possibility of a gift code.

what am I missing here

also the captcha is not a possibility when I last tried over TOR. it was SMS only

I was looking on my dashboard and seems that bitcoin is only available for the yearly payments.

Try switching to yearly and you should now see the option.

This is odd because my first monthly payment was effectively done with bitcoin (I wanted to try it out). This was some 10 months ago. Maybe it got changed in the meanwhile.

Sorry to hear about the TOR restriction.