Hey Liam,
Microsoft's documentation on ESU's through Azure Arc is a little confusing as they write examples of software licensing programs that provide Software Assurance, primarily targeting Enterprise scenarios. Microsoft's docs follow this trend as Enterprise are their biggest customers. Open Value is an eligible licensing program which offers Software Assurance, and you can use it for Azure Arc ESU compliance.
The program guide for Software Assurance states Open Value does provide what you need and purchasing a Windows Server 2022 subscription should cover existing Windows Server 2012 instances, as long as you purchase the correct number of cores.
Note that Software Assurance is strictly for compliance reasons and not technical reasons to enable ESUs for Azure Arc. You can always purchase them in Azure ahead of time, work with your licensing manager to ensure your targeted number of servers are covered with Software Assurance, and then assign the ESU licenses to the machines once they are enrolled.
If you want to follow the issue I opened with Microsoft to address this in the documentation, here's the link. https://github.com/MicrosoftDocs/azure-docs/issues/117876