cannot help with RSYNC but my setup is similar to yours so maybe there is some reference value for you.
until recently, my main NAS is a TrueNAS Core, and I back it up to a backup server (was another TrueNAS, and then fnOS NAS) via SyncBackFree. Problem free for years.
Then I flip’d the set up: my main NAS is a fnOS server backing up to a backup server (the original TrueNAS Core), also via SyncBackFree.
I do once a month and with remote on/off, I don’t even need to know where the servers are. convenient and super reliable.
Syncbackfree is a windows program. It has a variety of features but I use it to back up my key folders to a NAS server and from that NAS server to a backup server.
but depends on your use case, fnOS actually allows a variety of backup operations: it can backup to another fnOS server, to smb / ftp or a cloud account; or the other way around.
you can try it on a different machine, or try it as a VM on TrueNAS. I like fnOS enough to use it as my main NAS now (backing up to a TrueNAS Core server which was my main NAS until its replacement by the fnOS).
It does have its pluses and minuses, as everything in life