OK. Well, respectfully, I think it would be beneficial to find out more about how encryption, email servers, and encrypted messaging works. I think you're quite confused about the details here, and just getting a sense of the parts will help you in the long run. People use email differently - I don't use FB, so my main means of communication with family that is not Signal messages is email.

By "just use signal" I mean for sharing a password for a password protected email. Which you should only be sending to people you know already and can coordinate with. You're not sending password protected emails to random people or the government because it's not necessary for the reasons I explained earlier. If someone needs an encrypted message from ANYONE they will provide the method. Otherwise, they don't want encrypted messages and can't be trusted with data that should be encrypted.

Proton is secure, and I know because I had an old account I wanted to get access to and lost access to the recovery email, but had one on the same domain. I spent about a week doing back and forth emails with some guy who was trying to ask me to verify aspects of the account, which was my spam shield and dummy social media account and I hadn't used it for about a year. All he could see, when pressed, was header info: sender/receiver, date, time, ip address, sending agent. All things that are needed to route the message. It ended up being me able to confirm IP address and sending agent and access (I sent an email to my recovery address from an IP in this range on this date, last logged in on on this date, etc.). It was a pain for both of us.