10.0.19043.1023 Getting 0x800f0922 with KB5003637 Update, Even Offline

My C: drive is now a WD Black WD2003FZEX-00SRLA0, replacing the original WD2002FAEX-007BA0 which died of old age (failed to spin up one evening; SMART data was perfect). The other WD2002FAEX-007BA0 remains as a data disk.
For some time now I've had problems with about half the Windows Updates. They download and install fine, but on reboot the running progress number hangs at 100%. Rebooting allows undoing the update and recovery, and the Windows Update screen tells me that I have error 0x800f0922.
Until the KB5003637 update, my workaround was to download the update file and run it manually. Now, with build [Version 10.0.19043.1023], even that fails with the same error. Things I have done up to now:

1. Download and run KB5003637 update manually.
2. Check my partition sizes. My Recovery Partition is 529 MB and my EFI partition is 100 MB. On the Disk Management screen, the Recovery Partition is first, the EFI partition is second, and the rest of the 2 TB HD is the system partition with the Disk Management screen listing Boot, Page File, Crash Dump, Basic Data Partition for it.
3. Checked my App Readiness Service. It was OK, but stopped and restarted it anyway.
4. Rebuilt the Performance Counter settings with "lodctr /R" in ...\System32 and ...\SysWOW64, then resynced the counters with WMI using "winmgmt.exe /resyncperf"
5. Delete the fonts from my Recovery Partition (mount it as Y: using "mountvol Y: /s" and deleting all the fonts in EFI\Microsoft\Boot\Fonts from the command line).

My next step is to boot to GPARTED LIVE and expand my Recovery Partition to 700 MB. Before I do that, I thought that I would come here and look around, having had really great tech support here in the past with Windows 7 and Windows 10.

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After posting the message above, I was running the offline update again. It did appear to install OK this time, and the Update History shows that it did install successfully, but the Windows Update screen has an error message:

There were some problems installing updates, but we'll try again later. If you keep seeing this and want to search the web or contact support for information, this may help: (0x800f0922)

There is a "Retry" button. If I do retry, I'll use the downloaded version. That file name is windows10.0-kb5003637-x64_fd175a387e2f07586d85899a75d0bf10120a3c2c.msu (for checking to ensure that I'm using the right one).

I've already used information in all three of those threads. The batch file in "Reset Windows Update in Windows 10" batch file did not complete. I'll re-run it now. It wants to reboot. It did not succeed in stopping the encryption service; that's what stopped it before, but it continued this time. Since it continued, it may have completed its operation of resetting Windows Update.

This time, it did succeed in rebooting, but it failed to install. This time, the error is different: "Failed to install on ‎6/‎9/‎2021 - 0xc1900401." I've run SFC and DISM with no errors, and I did install update KB5003637 locally this time.

Looking at the messages in the update history, the security update installed but the 2021-06 cumulative update did not. Both reference KB5003637.

I've got a big, complicated system. My profile has most of it. I've got a new C: drive and run on an Aten KVMA switch, and also have a Synology NAS that I use to back up two other computers on my home network, and to exchange files between them. This computer has S: and Z: mapped to different folders on the NAS. My C: drive has just over 1 TB of data on it. The second HD has a little more. I've had problems with Windows Update failing about half the time for about a year now, and I've succeeded in making those updates work by downloading the files and running them locally, up to now.

So, what is error 0xc1900401?

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Troubleshooter for Windows Update finds no problem.

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I'm declaring victory at this point for two reasons:

1. The security update is installed.
2. Windows Update declares "You're up-to-date" and does not currently try to install the cumulative update.

I may be wrong, but it looks to me like Microsoft has recognized a problem with the cumulative update for some computers including mine and is staging the rollout to update these problem computers later.

If someone disagrees and/or knows what error 0xc1900401 is, please chip in.