What’s sad to me is that rsync hasn’t been touched to fix these issues in what feels like decades.

rsync does what was designed to do and the lack of scope creep is not a bad thing. There is "fpsync" - another tool on top of rsync (which was mentioned in one of the comments at article's page) that covers the parallel processing use-case: https://manpages.debian.org/bullseye/fpart/fpsync.1.en.html