I've had the same thoughts about Simplelogin, incredible service. Do you have a custom domain setup with it? If so, moving to self-host would be relatively easy. I don't have the time for that if something goes awry. If you have a custom domain with Simplelogin and switch email providers you can also do a catch-all until you can find a replacement, or with Tuta you can have unlimited custom domain addresses which gives you more control than a catch-all.
I also do not know the business model between Proton and Simplelogin, other than they have more access to infrastructure. Therefore a Simplelogin subscription alone will provide indirect funding to Proton, but I just don't want my funds going to Proton anymore. I'm moving to Tuta, but might need some time to think about the Simplelogin switch. Lastly, I've avoided Addy.io because its self-host is more involved to setup and I wasn't impressed with their service.