.NET 5 brings a professional .NET runtime to Linux-based PCs like the ComfilePi. The Blazor and Web Assembly technologies can be used to build GUI applications for the ComfilePi's modern web browser.
Blazor allows both client and server to be programmed almost entirely in C#. The System.Devices.Gpio namespace provides just about everything one would need to do IO on the ComfilePi. Blazor also includes support for real-time web applications that can be viewed and interacted with from multiple local or remote clients as demonstrated in the video above.
Visit .NET 6 Development on the ComfilePi for a detailed introduction.
Other GUI Frameworks that may also work, but have not been thoroughly evaluated:
The AdvancedHMI software allows you to create HMI's that are not possible with other off the shelf packages. The software is based on the .NET framework 4.x and uses the popular Visual Studio as it's designer. When developing with AdvancedHMI, you are creating a true executable that is quite fast for the Linux-based ComfilePi. Don't let Visual Studio mislead you into thinking it requires code writing experience because most HMIs are created without writing a single line of code.
Download the AdvancedHMI Visual Studio solution which contains a Modbus RTU driver and test project specifically for the ComfilePi.