A non-anonymous Signal account still gives you more privacy than an anonymous Proton Mail account.

All Signal can tell the authorities is the fact that your phone number was used to register a Signal account, the date (and time) that happened, and the date (and time) the account last connected to the Signal servers. In a country where you're allowed to use Signal, that information cannot hurt you.

Proton (and every "private" email provider), on the other hand, has much of that same information (when an account was registered and when it last connected to the server) plus the date and time each individual email was sent, its recipient, and its subject line. And if the email was sent to, or came from, someone who isn't using an encrypted email provider, the authorities will know who to serve a subpoena to to find an unencrypted copy of it. (!!!) That's all infinitely more dangerous than the knowledge that you have a Signal account. And all the authorities need to do to get it is find out your Proton Mail address.

It doesn't make sense under any coherent threat model to use "anonymous" (that amount of metadata would eventually deanonymise you anyway) "encrypted" email instead of Signal in a country where Signal is legal. You would be making yourself less safe for no reason.