I just spent the past three months building a custom GUI framework in C# using OpenTK, which is basically just a C# wrapper for OpenGL. It's inherently cross-platform because of .NET. [1]

After finishing, I regret spending so much time on it. For my use case, I think it would have been a better use of time to just use Electron. At least until .NET MAUI has Linux support, which is looking very promising.

[1] I haven't tried MacOS because I don't have a Mac and I believe it has OpenGL compatibility problems. It works great on Windows and Linux though.