is it possible / recommended to store the Library in a NAS, especially in a Synology NAS?
If yes, is synologies syncing mechanism CloudStationDrive supported / recommended?
It depends on whether macOS’s Core Data databases work with Synology. My guess is that they do, but I’ve not tried it. You could test this using the trial version of EagleFiler and see whether you get any errors.
Aside from that, it should work, but it would depend on whether you find the performance to be acceptable. Everything with a NAS is slower due to increased latency.
I’m not familiar with that syncing mechanism, but it should be fine so long as you only open the library on one Mac at a time and make sure that it has finished syncing before you switch to another Mac.