Connecting to a non free site (anti feature) doesn’t mean having proprietary or non free dependencies (being non FOSS), which is what you stated, but OK, you don’t trust proton proprietary services. I assume you don’t see youtube videos for the same reason, any FOSS privacy app or frontend will have that anti feature too.
Debian is the first choice if you look for stability and the last choice if you are looking for the latest versions of the software.