Best place to start is the github page, where the instructions are kept fairly up to date and @watchmeexplode5 has helped with that as well.
I think you can achieve that with with MergerFS. read up on 'FF' first found (I think) and just put your backup location first.
you can choose disk shares as your upload source. I don't think you'll get multiple copies, but I don't know if you'll waste bandwidth repeatedly uploading the same file. Why not just use a share aka Unraid so it doesn't matter what disk it's on?
Yeah, I guess it does save a lot of cash ;-). I've got over 0.5PB stored between media and backups which is at least £10k in drives, and that's before you factor in replacements, power, hassle, racks etc I was toying with putting up a donation or patreon or something on github as I read once they exist.
I've just created a paypal.me link if anyone wants to add to my beer fund for when there's a bar I can visit!