Storing disk images is probably not the most convenient or efficient way to go. If this was ZFS you can send incremental images so only the changes are transferred, but it’s still a very storage-intensive way to backup your data if not all of it is important to you. For example, most people would be able to rebuild their movie collection so protecting may not be as important as it would be for family photos.
Syncthing is an open source directory synchronization program that might work for you. You install it on your NAS and PC, run it in the background, then pair them and you can sync folders in real time. There is a “send only” option you could enable on the NAS so it will ignore changes the PC makes to the files if you want to make sure the PC doesn’t boark the files. There is a community package repository for Synology devices that has Syncthing in it, or so I hear. Syncthing struggles a bit with >100GB files. Syncthing has versioning settings that can archive modified/deleted copies of files if you want. I use syncthing on all my devices to keep copies of my stuff on all my devices and it works great for what I need.
If you can’t get Syncthing working, Resilio Sync is functionally very similar but has a freemium pricing model and isn’t open source, so it has some annoyances.
In terms of encryption, if you configure encryption of the HDD partition in your PC’s OS, then it would be encrypted on disk but the PC OS would still be able to see the contents of files once you log in. Bitlocker is the easy button solution on Windows, and Linux has a plethora of disk/partition/file encryption schemes. Many of these would work with Syncthing.