This is a misconception. If the sender is outside proton mail the emails arrive in plain text. So it is possible for proton to read those emails. It is just that they pinky promise not read them and immediately encrypted them with your key. But if they wanted to they can read and moreover SMTP means your email has already traveled through multiple MX which could read your email but This is mostly not an issue since most email provider do encrypt with SSL of receiving MX but you might want to check few services use very very outdated softwares. But keep in mind SSL encryption is with Proton’s keys and by necessity they have to first decrypt the SSL encrypted email and then encrypt with your key
Snap doesn’t just bring lots of hate in comments it also brings a lot of bloat in your system which is a big no in Linux community. Another thing is canonical is going out of their way to force snap. In Ubuntu even if you do apt install it is installing snap packages.
I’m not sure if there are more snap packages than flatpaks or .deb/.rpm but most Linux users are competent enough to either add custom repos or follow simple build instructions to build from source.