@david-wsd great questions! I believe you are already familiar with dockable panels (for those that aren’t , an example is here: sdk/Examples/PluginDevelopment.Gui/GUI/DockablePanel.cs · master · OpenTAP / OpenTAP · GitLab)
This is one possibility for customizing a UI without doing too much UI work. We have even recently enabled something we initially called “Focus Mode” which allows you to hide certain aspects of the UI for simplicity (press Alt+Shift+Enter in 9.12+).
For integrating with C# API (including starting OpenTAP, loading plugins, settings, and Result Listeners) you can look at this example. It doesn’t go into details of how to create a UI, but it shows the OpenTAP APIs you would need:
If you are looking for more of a web interface there is also the REST API Plugin. This is built on OpenAPI/Swagger and provides a really nice interface for web UIs.