I didn't read the whole thing, but couldn't you just rsync over ssh from either end?
Your suggestions is also very nice, you can just turn off your own PC and wait for it to finish.
Synology actually supports the old (non-encrypted) rsync protocol, too. On a trusted LAN, it runs much quicker that way than with rsync-over-ssh...
I just finished slurping all of my files off an old Synology and onto a TrueNAS server. Unlike the article here, my Syno is 2x1Gb in a LAG, so I was really only getting ~1Gbps at max.
One disappointment with TrueNAS is that it doesn't support SMB-MC unless you put each interface on a separate subnet (which is a strange limitation because Samba itself doesn't seem to suffer from this, since synology handles it fine)
The situation in the article is one NAS is 10 Gbit and one NAS had 4x1Gbit. SMB will use MultiChannel to aggregate the 4x1GBit ports, but rsync will just pick one and be limited at 25% of the available bandwidth.
I don't even think rclone can split across multiple interfaces like SMB does.