Update to my previous post and some more info to help others with the same issue and perhaps come up with a solution:
Running a Time Machine backup to an empty 4T external drive and then a different brand 8T external, the pattern was repetitive- it ran fine until it hit the same approximate point every time- about 370 GB backed up and about 600 GB remaining. Then, when I resumed the backup it crashed repeatedly within a few minutes, I resumed it over and over and the same result. These crashes were all the same- first, "backup stopped," then cursor froze, then a black screen, then a restart and a crash report.
Next, I tried backing it up to my 3T Time Capsule via WIFi with nothing attached. Total size about 970 GB, time- 15 hours. It ran for about 10 hours and restarted with no crash report. After it restarted, I resumed the backup and it completed.
After all attempts at a backup had restarted or crashed, I tried just letting it sit overnight with nothing running and nothing attached. In the morning I found that it had restarted again at some point - no crash report though.
After hours with Apple Support he recommended and made a Genius Bar appointment at which the store's top tech guy and 3 others spent an hour with me. Lots of speculation- hardware issue, software issue, problem with USB-C data transfer, problem with external drive, wipe and reinstall and a few more. Consensus was to return it. A store return was said to be impossible because I had ordered it through Apple Business, so I waited 2 weeks for an RMA label and finally got it after many hours of emails, phone calls and frustration. While waiting, I wiped it and reinstalled the OS to try to eliminate a possible corrupted OS and multiple errors occurred during the reinstall and the update of iTunes and after setting it up with just basic included apps, Photos crashed 5 times in 10 minutes.
At that point, I gave up, waited for the RMA labels and finally returned it yesterday.
I did order another one right away, but was informed first that shipping would be 4 to 5 weeks. Later was informed that it would be 7 weeks or so and later that it was on backorder.
I'd like to add that I also purchased a 13" model MBP, 1TB SSD and top of the line on everything else and it works perfectly and I love it.