I couldn't tell you if it's the right way but I used it on my Rpi4 to sync 4tb of stuff from my Plex drive to a backup and set a script up to have it check/mirror daily. Took a day and a half to copy and now it syncs in minutes tops when there's new data

yes, it's the right way to go.

rsync over ssh is the best, and works as long as rsync is installed on both systems.

It depends

rsync is fine, but to clarify a little further...

If you think you'll stop the transfer and want it to resume (and some data might have changed), then yep, rsync is best.

But, if you're just doing a 1-off bulk transfer in a single run, then you could use other tools like xcopy / scp or - if you've mounted the remote NAS at a local mount point - just plain old cp

The reason for that is that rsync has to work out what's at the other end for each file, so it's doing some back & forwards communications each time which as someone else pointed out can load the CPU and reduce throughput.

(From memory, I think Raspberry Pi don't handle large transfers over scp well... I seem to recall a buffer gets saturated and the throughput drops off after a minute or so)

Also, on a local network, there's probably no point in using encryption or compression options - esp. for photos / videos / music... you're just loading the CPU again to work out that it can't compress any further.