i also have a genuine synology ds918+ and tried copying the same but no problem even i install openmediavault no problem.
I looked on intel.com and found out that this i225v card has a driver error, intel has released a new update on windows but I don't know if it's available on linux.
https://www.intel.com/content/www/us/en/support/articles/000057261/ethernet-products/gigabit-ethernet-controllers-up-to-2-5gbe.html
Automated RedPill Loader having a network connection error with intel i225, when i copy many files about 50gb from pc to nas, it runs 20%, then i lose network connection from nas, i can't access nas anymore i have to turn off nas manually to get in.