I forgot to come back to this community to update this post, but news that the Murena Fairphone 4 is now available in the USA has brought me back.
While I personally only use apps from Droidify (modern F-Droid app store with auto-update functionality), and there are no FOSS visual voicemail apps available, I think this is a solution that would be acceptable for most people. I don’t consider visual voicemail to be very important, so the lack of it doesn’t bother me.
I haven’t had issues with the proximity sensor in a while (months?). I don’t know when the issue was fixed, but I am glad it is.
There are no more unresolved issues with this phone.