I had to report my findings, I had install 2026 on a new machine. I can report creating a new add-in seemed to work with the ArcGIS Pro 3.5 SDK along with debugging an existing one. However, I have had one annoyance where I'd get the error below every so often when rebuilding a solution. But a subsequent build works fine.
1> Failed to load the dll from [C:\Program Files\dotnet\host\fxr\10.0.3\hostfxr.dll], HRESULT: 0x800700C1
1> The library hostfxr.dll was found, but loading it from C:\Program Files\dotnet\host\fxr\10.0.3\hostfxr.dll failed
1> - Installing .NET prerequisites might help resolve this problem.
1> https://go.microsoft.com/fwlink/?linkid=798306
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========
========== Rebuild completed at 08:20 and took 05.551 seconds ==========
