But you want to use your existing back-end and GUI code as much as possible. You just want to image against a different window system, instead of OpenGL.
So, I’m thinking you can use the same standard .NET libraries, but have it imaged on the Web, as is done with iOS and Android. So, Web support for the front of .NET. They already have a WebView in MAUI, so you already have Web support on the back of .NET. That’s kind of useless if you didn’t start with a Web app. And MAUI does not have Linux support, except for Android, so perhaps that’s why you’re not seeing a native Linux client.
But… Again, ARC seems to suggest that Roon has invested in Flutter & Dart. Flutter does have native Web support. What’s more, it has migration tutorials for Xamarin.Forms developers, to move their apps to Flutter.