Another user said it - what your asking for isn't a backup, it's just data transfer.
It sounds like you're looking for a storage backend that hosts all your data and can download data to the client side on the fly.
If your use case is Windows, Nextcloud Desktop may be what you looking for. I have a similar setup with the game clips folder. It detects changes and auto uploads then, while deleting less recently used data that's properly server side. This feature might be in Mac but I haven't tested it.
Backup wise, I capture an rsync of the nextcloud database and filesystem server-side and store it on a different chassis. That then gets backed up again to a USB drive I can grab and run.