remote:windows/NAS (timestamp) backup

I think the best approach is to back up the data to another device.  For instance, you could back up the NAS to a USB drive weekly, detaching the drive when its not in use.  

But snapshots can help if you have enough free space on the NAS.  If you have snapshots enabled on the backup share, then when the malware hits the old unencrypted versions will be migrated to the newest snapshot.  You can then roll back to the unencrypted version.

However, since snapshots are automatically pruned if the volume becomes more than 90% full, you do need to make sure you have a enough free space on the NAS. If the entire NAS is potentially vulnerable to PC-induced ransomware, then you'd need to make sure the NAS is never more than 45% full.  

If only some of the volume is vulnerable, you can use less - the formula would be free space >= 10% of the volume size + the vulnerable data.  For instance, if the NAS has a 3 TB volume, and only a 500 GB backup share is being guarded from the malware, you'd need to make sure there is always at least 800 GB of free space (and that snapshots are off on other windows shares).