Hi Doug,
The .NET core does not have full designer support for custom controls to
customize the settings through smart tag and interaction over the controls in
the designer. You can find the details in below GitHub query,
GitHub Link: https://github.com/dotnet/winforms/issues/4887
Also,
they have added designer works in WinForms roadmap. Please refer to the below
link also,
https://github.com/dotnet/winforms/blob/main/docs/roadmap.md

As this feature is not available in our product, we have already
added it to our feature request list, but we don’t have any immediate plans to
implement this feature. At the planning stage for every release cycle, we
review all open features. We will let
you know when this feature is implemented. The status of implementation can be
tracked through our Features Management System.
https://www.syncfusion.com/feedback/19702/need-to-provide-smarttags-and-designer-support-for-ribboncontroladv-control-in
If you have any more specifications/suggestions to
the feature request, you can add it as a comment in the portal and cast your
vote to make it count.
Regards,
Subashini M.