Basically all email is E2EE already since SSL/TLS is usually used for transport, even gmail and similar. But encrypted at rest in theory would help with stopping people from reading emails off the server.
You also have to trust that Proton truly doesn't have your keys to decrypt, but I imagine they do since you just login with a username/password combo and that's enough to decrypt the emails.
Although I don't think it matters that much, my email is basically receiving notifications from services I use and occasional emails with a friend about planning a trip or something like that, nothing that particularly needs to be super private, just using a mail provider that isn't actively scraping my data for ads (aka; gmail) is enough for me.
For private communications I would use something more suited to that, like any of the reasonable E2EE chat apps.
For my area it wouldn't really be possible, there's maybe 1 place that regularly has gluten free stuff, but the rest need real-time data pulled from their facebook or something because it changes daily/weekly what they offer.