Помните, что в .NET Core 3 появилась поддержка Windows Forms? https://devblogs.microsoft.com/dotnet/introducing-net-core-windows-forms-designer-preview-1/
Microsoft News
Introducing .NET Core Windows Forms Designer Preview 1
Introducing .NET Core Windows Forms Designer Preview 1 We just released a GA version of .NET Core 3.0 that includes support for Windows Forms and WPF. And along with that release we’re happy to announce the first preview version of the Windows Forms Designer…