CPU and RAM are not the only limiting factors. Not only that but not everything runs multithreaded. Maybe some piece of the puzzle is not multithreaded and is using all it can from a single core (assuming that cpu is multi- core)
Depending on how much you value your time, you’re almost certainly better off getting a new machine to run pfsense.
I have found synching to be very useful for making copies of files across devices. I have it setup to mirror photos from my phone, photos from my wife’s phone, and various other things (to-do lists for todo.txt, notes and shopping lists for obsidian… stuff like that) back to my desktop and my NAS. You can set it to do one-way sync (which is more like a backup) or two way sync (where changes anywhere are propagated to everywhere else).
As others have said, it’s not really a true backup solution, but handy to have immediately accessible copies of what’s on your phone in case of phone loss or damage.
For photo viewing and sharing, I am more or less pointing the photo sharing app on my NAS to the photos I sync from phone. They all get dropped into an “inbox” when first synced and then can be organized from there.
You may also want an actual backup solution. There are quite a few and that’s a different topic. The reason I bring it up, though, is that simply mirroring what’s currently on device is not considered a real backup by most people, and for good reason.
“Sorry, disc must be scratched”
Looks like you already settled on this, but I’m doing exactly this (syncing obsidian, as well as photos/videos from camera reel), to desktop and NAS, using syncthing-fork. Let me know if you want some pointers.