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.
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.