The "probably Linux" line was when I was thinking of using .NET MAUI for the GUI, and they were considering adding Linux support. My bad, I need to change that.
.NET MAUI has decided not to do that, so I'm going to build the GUI with Avalonia, which does have Windows / MacOS / Android / iOS and Linux desktop support, as well as WASM support, all from the same code base.
If I weren't using a cross-platform GUI framework like Avalonia, I really might not bother with a Linux GUI, or at least deprioritize it; I expect most Linux devs will just use the CLI. I'm totally curious how much GUI usage there will be on Linux...