Hosting your own mail server. But unless you're really certain you want to do that, and are very clear on what you need to do to keep your setup secure and your mails out of spam, don't. It's not impossible, but probably not worth the effort.

I am personally probably going to switch to mailbox.org when my Proton contract is up, already using it with an org I sysadmin for, and all in all pretty happy. Of course no guarantees that they don't enshitify, and they recently raised prices, but they're a lot less publicity/marketing focused compared to Proton. Less silicon valley energy, if you get me.

I am also setting up my own Mailserver with a throwaway domain to test the waters, but I'm not expecting to use that for anything serious in the near future.

As I've said elswere, you can let Proton know your thoughts in their community voice platform/forum. You can also email thier foundation.

I use: 1984.hosting

100% green energy, small family company. I have never had any sort of problem.