Time Machine over NAS has always been unstable for me. As you experienced, my backup would be regularly corrupted and would need to restart. Alternatively, a normal <10G backup could take in excess of 24h.
I eventually gave up on the NAS and now backup to external drives at home.
I eventually gave up on the NAS and now backup to external drives at home.
This site has excellent articles on Time Machine - scroll down to Time Machine Problems: https://eclecticlight.co/mac-troubleshooting-summary/