One of the issues I have with email is that it is used for everything: things that might be urgent/time sensitive, complete garbage, but also a lot of stuff I want to read, but when I have the time, not when it is received. I have found that moving a lot of the non-urgent stuff to RSS feeds has decluttered my email inbox, drastically reduced the number of unwanted notifications and actually lead to me reading more of the things I wanted to read because they show up in the right place at the right time: the RSS reader. When I am using the ProtonMail app, it's usually because I'm busy trying to get stuff done, which isn't the time for distractions. When I am using the RSS reader, it's because I want to read and have the time. Most RSS readers have options for some sort of auto-delete mechanism with some more flexibility than just the 30 days option Proton has. Some of the more advanced readers even allow for all kinds of automation/post processing. Right now I am using https://kill-the-newsletter.com, which is an awesome (free/donation based) service that converts emails to RSS posts. It works great, and if this feature isn't implemented I will happily keep using my current setup, but I really feel that it improves the email experience so much that it deserves be integrated in the same way SimpleLogin has been integrated. It is open source (repo: https://github.com/leafac/kill-the-newsletter). You will need some kind of filtering in the same way the forwarding option has. Of course, just like with forwarding, E2EE will have to be disabled for that address, but the messages you would forward to RSS are usually not of a sensitive nature. RSS has absolutely no pretense of being secure, but as long as users are aware of this, they can decide for themselves if that's acceptable for their use case.
RSS is one of the fundamental open protocols that can help us move away from the data-hungry, walled garden, privacy nightmare tech companies that have all but ruined the internet.