I don’t have a Synology NAS but I did used to suffer from the stutters and delays you mention for UHD,
the cure for me was to set up fstab for my SMB shares configuring fstab
Since I took the time to set that up (it was much easier thanks to that tutorial than I was expecting) my GB ethernet adapter has been consigned to collecting dust in my tech drawer, and I watch my UHD content glitch free via the “weedy” 10/100 ethernet port built into the Vero