No, you don't need forms. If you're adding a reference to the Advantage DLLs, you add them both (ACCPAC.Advantage.dll and ACCPAC.Advantage.Types.dll) from here:

C:\Program Files (x86)\Common Files\Sage\Sage 300 ERP\

Then see my other response to you regarding creating a session.

Thanks, so I think I figured it out. I'm using .NET 5 and it won't automatically pull Windows.Forms from GAC like .net framework apps do. Basically, it seems I may not be able to use .net core+ w/ Advantage.DLL

I think you're right.  The last time I used the Advantage DLL was a VB6 project in 2005.

You can't use .NET Core with the Sage .NET API because they are Win32 DLLs and .NET Core is platform independent. Trying to reference the Win32 DLLs from a .NET Core project will break it because it will no longer be platform independent. You need to tell the project that the target platform is x86

You can't use .NET Core with the Sage .NET API because they are Win32 DLLs and .NET Core is platform independent. Trying to reference the Win32 DLLs from a .NET Core project will break it because it will no longer be platform independent. You need to tell the project that the target platform is x86