First, thank you for your help. Here’s an update.
By checking the box “NFSv3 ownership model for NFSv4” in my TrueNAS server and restarting the service, all of the directories (except for .dietpi-sync.log) came under the ownership of truenas_dietpi
truenas_diepi is a user that I set up on both machines, so this is good. Unfortunately, I ran dietpi-sync again and still get the Invalid argument (22) for multiple chown lines.
I also found the user management link you posted in a different thread very helpful. This is what I wanted to learn. Thank you again.
Still digging, but wanted to post an update.