Also a hobbyist sysadmin at home, but after reading a few comments on the internet, I found that the rsync -z compress flag was bottlenecked on a maxed-out CPU thread of my Raspberry Pi 4 NAS transfers.
Admittedly, these are mostly "local" drive-to-drive transfers over USB rather than server-to-server transfers over house-wide gigabit. But consider trying the transfer without compression.
I'll definitely have to read up on your other flags though, if your rsync works from just one machine rather than both that might solve a minor problem case I have where the destination machine lacks rsync...
Admittedly, these are mostly "local" drive-to-drive transfers over USB rather than server-to-server transfers over house-wide gigabit. But consider trying the transfer without compression.
I'll definitely have to read up on your other flags though, if your rsync works from just one machine rather than both that might solve a minor problem case I have where the destination machine lacks rsync...
https://unix.stackexchange.com/questions/188737/does-compres...