Have you tried Carbon Copy Cloner by bombich.com ? There is a free one month tryout and that is an excellent app. It's a great macOS gui for rsync.

From my experience, Carbon Copy Cloner as a rsync gui makes better use of the bandwidth available compared to copying using the Finder's drag and drop. If you don't want to use the NAS interface, that's the only way to maximize the transfer rate.

I will also suggest using Menumeters to measure your network speeds, and use a wired Ethernet connection instead of Wifi if this is applicable.