disroot web mail service is a reasonable choice as it is fully opensource and run by the community for the community. It has a pretty well IMAP support.Since it is fully funded by the community sometimes the server can go down for long time (Hardware cost is high due to dam ai and greedy corporate) so be aware of that also If your using it for buisness make sure to donate to them and apply for extra storage request and custom domain support is available.Also you have to wait for approval while registering due to the spam they are undergoing. One important thing is it lacks e2ee but its privacy focused so no need to worry.They even have an .onion site.Their website is disroot.org. Lastly the registration will not be available during the week ends.I Forgot to mention that the disroot app(mostly user maual) is available only in android fdroid or apk no play store app(which is good).
I also don’t like to run a bridge between thunderbird and proton but the e2ee doesn’t work for IMAP directly so the bridge encrypts your email before sending.Tuta doesnt even have IMAP support, you have to use their desktop app for emailing but I prefer the web version. Remember