sync software suggestions

I have a laptop with Mint and Fedora; a PC with Mint, Fedora and Windows 10. I've gone through a lot of reviews on suncing software and haven't come up with anything. I'm not against paying for the service, but would like a one time fee and not monthly. I have Insync now and it's not too bad, but uses MS One drive and Google drive. I would like to get away from being tied to either and just use a dedicated file manager.
TIA.