Hello, thomasblofield.
Welcome to Microsoft Community.
I have some ideas.
Please try the add feature in the control panel.
The method of manually downloading and installing packages from my memory has become obsolete since Windows 10.
Press Win+R to open Windows Run -> Type "optionalfeatures" and open it -> Check ".Net framework 4.8" -> Wait for the installation to complete -> Restart Windows
Here is the documentation I found:
Install the .NET Framework on Windows 10 - .NET Framework | Microsoft Learn
If it doesn't work, please try the methods mentioned in the document.
If there are any updates, please feel free to reply.
Best regards.
Malygos - MSFT | Microsoft Community Support Specialist