Unable to activate SQL Server Agent after upgrade from Express to Standard

I have just upgraded a MS SQL-server Express 2019 to Standard using the documentation from Microsoft here: Upgrade to a Different Edition of SQL Server. Everything went well and looking at properties for the Instance shows that it has kept the instance name SQLEXPRESS but under Product it shows Microsoft SQL Server Standard (64-bit). The users can connect as usual, so I know the databases are up and running as they should as well.

However, Step 13 in the documentation says:

If you upgraded from SQL Server Express, you must perform additional steps before you can use your upgraded instance of SQL Server:

• Enable the SQL Server Agent service in Windows SCM.

• Provision the SQL Server Agent service account by using SQL Server Configuration Manager.

I'm not having any luck with this step. I tried following the guide here: How Do I Enable SQL Server Agent Service?. But when I change the Log on account to Local System as recommended in the article and try to set the service to Automatic I get a WMI Provider error 0x80070422. I've also tried setting it to use the Network Service and Local Service account.

I've restarted all the SQL-server services and I've rebooted the server it's running on (Windows Server 2019 Standard) in case there was something that needed to be refreshed for the installation to fully identify as a SQL-Server Standard.

If I try to go via the Services MMC instead of SQL Server Configuration Manager I'm able to change startup type without getting any errors, but the Service still just starts and stops. In Event Viewer -> Application it shows up as Event ID 101 - SQLServerAgent service successfully started. And Event ID 102 - SQLServerAgent service successfully stopped.

As far as I can tell these problems seem to be related to permissions. I saw in one place that you could try to use the same user as the SQL Server instance (MSSQL$SQLEXPRESS), but I don't have the password for that user/service account as I was not the one who installed the server in the first place. And I'm loath to try to change that password in case it's used somewhere else and breaks something.