Dear @William Fiske
If the update immediately jumps from 50% to 100% but does not appear to install correctly, I would first check whether the update is actually failing or whether Windows Update is simply reporting the progress incorrectly.
You can check the currently installed Defender platform version by opening Windows Security > Virus & threat protection > About and looking at the Antimalware Client Version and Engine Version.
You can also check the installed package from PowerShell by running:
Get-MpComputerStatus | Select-Object AMProductVersion, AMServiceVersion, AntispywareSignatureVersion, AntivirusSignatureVersion
If the installed Defender platform version is already 10.0.29628.1000 or newer, there may be nothing else you need to install manually.
You can also check the Defender operational log under:
Event Viewer --› Applications and Services Logs --› Microsoft --› Windows --› Windows Defender --› Operational
This should give you a better indication of why the platform update is not being applied.
If you can provide the Windows version/build, the current Defender platform version, and any error code from Windows Update, it would be easier to determine what is preventing KB5007651 from installing.