Using TM on a 3rd party network server has always been - well problematic would be one word for it. I’ve used a Synergy NAS for quite some time and I won’t go into all the issues, fixes, updates, and hair pulled out. The bottom line is that if you are lucky TM backups over a NAS will work fine until a MacOS update or upgrade breaks it. Then, if you are lucky an OS update for your NAS will fix the problem until the next time. That’s if you are lucky. If you are unlucky your TM backups will stop updating for some inexplicable reason and starting all over will be your only solution. So, my backups on the NAS aren’t TM backups, I use a 3rd party app. I also backup to 2 external SSDs using TM because I like to keep a backup offsite.