Very sorry for your loss. Hopefully it’s a config issue stopping the zstick from showing up, and is not completely corrupt. Synology makes it very difficult to pass usb sticks through. It’s possible you can change settings to get it to show up again. Check this thread for synology config ideas
If the zstick is the problem (and not synology config) what is the model zstick you are dealing with? Aeotec has some backup tools, but it’s model dependent.
There is also a silicone labs program that could backup the nvm, but it’s not very user friendly.
All would require access to the zstick though.
I would use zwavejs2mqtt (now zwavejsui) over just zwavejs since you have access to more config options, including nvm backups.