+1 for Syncthing. If all you want is files syncing between PCs, it's simple and effective. My only real complaint is the way you interface with it. Every client is also a server serving up a web interface. The benefit is that you can manage all your machines from whatever machine you are on, the bad side is you have a bunch of web servers being open in your house, and have to maintain a bunch of usernames/passwords. If you have a good password manager that isn't too much overhead though. And if you use the "introducer" feature, making your NAS the introducer, then every new computer you add will propagate to all the other ones.
For a more user-friendly (IMO) alternative, altough not open source, there is Resilio Sync. The main difference with Syncthing is that you don't introduce clients to each other; you just share folders.
As far as I know, Syncthing's local web interface is locked down by default. You should only be able to access it from localhost unless you change the settings. That said, there is some security concerns about JS in browsers making web requests to localhost, which is why I keep a password on Syncthing even though the webui isn't remotely connectable.