@paperdigits, I already know git-annex and plan git-annexing my photo library for ages, not only for the checksums but copying files around etc. I already tried it 2 times but failed, one time the problem was that I was not able to install the same version of the software on client and server, and two different annex versions were not able tu talk to each other in a useful way. The second time, the problem was that I tried to set it up on an already existing git repo, which was synced with unison to the server. That messed both repos up to a degree that I was not able to fix, and that’s the status quo for several years now. Luckily, I am still able to use the git same git repo for the sidecars.
That written, it does not mean git-annex is a bad software, I like it very much and use it in places where I setup something new or handle less data so that I can copy things over if I do something stupid. Furthermore, both cases were mistakes done by me and not the software, and, time permitting, I will do another try.
That said, I still think additional checksums are cheap and therefore a good thing to do, and, furthermore, a better integration into my workflow would be extremely helpful.