Since my post, someone else has replied and said they were having the same problem. Your valuable ideas may help people coming in looking for answers, so thanks much for taking the time to reply. In my case, no ideas in your first paragraph apply. As far as external drives go, I bought identical G-tech 5Ts when I bought my new M2. I back up with both each week. (In my very long history with Macs, one time, a long time ago, the internal hardrive and external hard drives died on exactly the same day. It was disastrous. Now I back up twice every single time.) Because I have 2, I tried both and switched to many different usb-c cables. All combinations were just as slow.
The only thing that seems to have made some kind of difference has been my disabling Spotlight indexing. I'm unhappy that I can't use Spotlight any more but indexing my files for quick searches slows down my read/write speeds. I disabled it on my Mac following this method: System Settings > Siri & Spotlight > Spotlight Privacy… > There I added my hard disk and my external drive to the list of excluded items.
