If it were me I would copy via the network once the new psu arrives.
Have a look at rsync command via the terminal on one of the Mac’s.
Set the mac not to sleep while plugged in.
Hardwire the Mac and the nas’s for maximum throughput.
You can then look at copying folders rather than the whole file system, this makes trouble shooting and confirming data integrity easier to complete as your working on smaller chunks.
You might find the qnap has a rsync client built in if it does it would be quicker as it removes the network hop of the Mac.
Once you’ve finished, give some thought to how you’ll back up the qnap as with 4hdd instead of 2 you’ll have the potential for much more data to manage.