Change Configuration Manager Site Server OS – In-place Upgrade Reference

Hi rejohnson!

To your first question - here I need details to the Environment - does the Site Server also host the Software Update Point Role, or did you shift this Role to the other Server from the beginning - Is it a Shared SUSDB-Config? Because if the Site Server does not hold the Role you can do the Inplace Upgrade - you only need to ensure you have the WSUS API / Console-Tools installed to establish a connection to the SUP.
Site prerequisites - Configuration Manager | Microsoft Learn

As Anthony mentioned you need the latest Service Pack on SQL 2012 before doing the Upgrade. 
Supported SQL Server versions - Configuration Manager | Microsoft Learn

It is Supported to Upgrade from SQL 2012 to SQL 2019 straight ahead. However the PreReq for that is that you have at least Windows Server 2016 in use.
Supported version and edition upgrades (SQL Server 2019) - SQL Server | Microsoft Learn

After the In-place Upgrade I recommend to upgrade SQL as soon as possible. Like in the document mentioned have your Backups and DR-Plan ready - better safe then say sorry. Another Option for SQL would be to move the database to different SQL-Server. Do the In-Place-Upgrade of Windows, install the latest supported SQL-Version to your ConfigMgr-Environment. Then move the SQL-DB back, by Backup/Restore or Detach/Attach approach.