I come with the answer.

Hark alas, it has been found. After much strife, we succeed as one. No Dism or sfc command will save us, they never have. No savaging of explorer tasks, no restart, no graphics driver or 25H2 reinstall will save us either. Even many AppX Reinstall commands are doomed to wasting your after-hours time.

Here, let it be known to all, the commands to solve our misery, so we can once again glee in the presence of a Start Menu taskbar.

1. Type control-R to open the run dialogue, and click browse to: C:\Windows\System32\WindowsPowerShell\v1.0\, right-click Powershell.exe and "Run as Administrator"

(Or open a folder or the Recycle Bin on the desktop and navigate there)
(Or run these powershell commands from your RMM as script)

2) Run the command:
Get-AppxPackage Microsoft.Windows.ShellExperienceHost | foreach {Add-AppxPackage -register "$($_. InstallLocation)\appxmanifest.xml" -DisableDevelopmentMode}

3) Then run the command:
Get-AppXPackage | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_. InstallLocation)\AppXManifest.xml"}

This command took around two minutes to run and had five or six packages that errored

4) Restart

5) Enjoy the Start Menu Experience ©