Hi @Nick Ryan ,
> stopped and restarted SSMS (2016)
As fa as I know, last supporting debugging SSMS version is 17.9.1, suggest you using it, you can get it from here. If it is still not work, you can easily connect to SQL Server from Visual Studio and perform Debugging on Stored Procedures after connecting the server. Please refer to below blog to get detail steps.
How to Debug Stored Procedures in SQL Server using Visual Studio 2019
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