Yes I do, great testing!
I suggested trying the hasher in this post, I guess it can take much of the load from the SFTP server - especially while checking.
An ordinary rclone sync like yours should not be able to make the Finder freeze unless there was a major memory leak in rclone, but we already know the rclone memory usage is within expectations and low.
My immediate thought is therefore that the sync is pushing some (unstable) part of the Mac OS over the edge. It could be anything, but my immediate suspicion in your setup would be the something related to networking, e.g. your network driver or high packet loss on your network. This is because you already tested that high concurrent CPU and disk usage didn't trigger any issues while trying rclone check.
I therefore suggest you start ping IP.OF.YOUR.NAS in a sperate terminal during your sync and then monitor it up to and during a freeze.
Does your Finder show networked drives e.g. from the NAS?