Have your looked at chocolatey?

I can vouch for WAU. Been using it for over a year now on 8k+ PC's for around 100 different apps. Using app whitelist via gpo/intune to ensure only apps we have tested are included in updates. You may find challenges with some apps that have custom things like network license servers (JetBrains) not correctly working with this automation. Some apps are tricky as they aren't capable at system level since they install into %appdata%, so we install those at the user level deployment in sccm.

One issue I see is that if you are on win11, these updates don't start processing until at least one user has logged into the system. This is a Windows issue, not WAU. These deployments start installing as soon as they login and the wait times are pretty short.

I know you said open source and free, but PatchMyPC is cheap if you have WSUS or Intune already. Otherwise chocolatey is the closest to your request.

Side note, you can use psexec to span a power shell as system to work with it interactively to understand what’s going wrong. I’m guessing you either need to import a module or make sure the correct powershell version is being invoked.

Don't do this

You need a proper endpoint management solution