Unable to run Software Update on 2019 iMac 3.6 GHz

Because this iMac has such a slow hard drive, I was using an OWC Envoy SSD external to boot this computer. I was running Monterey on that SSD and keeping it updated and everything was working fine. When I tried to install the latest Monterey update, the iMac decided the Envoy was an incompatible drive. I couldn't get past the incompatible drive screen.

In troubleshooting, I reset the NVRAM. That put me back to booting from the internal original 1TB hard drive. Which was an improvement because all I was seeing was incompatible drive notes being displayed on the screen.

One hint I saw in Apple Community is that the internal hard drive has to have the latest version of Monterey so that it can recognize the external SSD.

The internal drive is running Catalina 10.15.4, I never updated it because I was using the external drive to boot. The external drive was running Monterey.

I have been trying to update MacOS on the internal from Catalina 15.4. Using Software Update results in a constantly spinning checking for updates (left it running overnight -- same spinning wheel).

I thought perhaps the OWC drive still being mounted (I could see it in Disk Utility, but it doesn't show up in Finder) was causing problems with upgrading the internal drive. I couldn't eject the OWC because Disk Utility said it was still in use (UPDATE dmg on the desktop I guess). I shut down and removed the OWC disk and rebooted. The UPDATE folder disappeared from the desktop.

I tried going directly to the App Store and downloading Catalina first to check that I had the latest version. Clicking Download results in the Software Update box opening with a solid blue line and Finding Update displayed. But I don't see any actual download happening. I then tried downloading Big Sur with the same results. I'm leaving that to run overnight too, but don't expect anything to change.

Any suggestions?