Partially, you need to rely on the community for GUI stuff (Avalonia and Uno), and old Microsoft still pushes VS/Windows as the best experience, anyone else that wants a VS like experience has to buy Rider.
Yes, there is VS Code, which besides being Electron based, Microsoft is quite open that will never achieve feature parity with VS.
With regards to UI there is Microsoft’s MAUI, which I personally prefer over Avalonia. I love the single project approach of MAUI. I think Avalonia also relies on MAUI controls to some extent (I seem to recall a <UseMaui /> project setting in Avalonia projects.
MAUI doesn't count if "supports GNU/Linux" is part of being considered FOSS proper, and on macOS they took the shortcut of using Mac Catalyst instead of macOS UI APIs.
Yes, there is VS Code, which besides being Electron based, Microsoft is quite open that will never achieve feature parity with VS.