.NET is one of the easiest to install and robust cross-platform systems out there. It's actually very impressive. I don't know of a single other platform that nails cross-platform so well.
The effort of .NET Framework to .NET Core to .NET 5 and now up to .NET 9 is well over a decade long of steady and increasing progress.
For (2), this is often misunderstood and mistated.
The JVM includes no GUI framework. .NET also includes no GUI framework. .NET happens to have a specific framework for Windows. But just like the JVM, there third-party GUI frameworks such as MAUI, Uno, Avalonia, and others.
So no, there's isn't a "large part" of .NET that isn't cross-platform.
> 3. the level and quality of official tooling provided for Linux and macOS is incomparable to their Windows offerings
The effort of .NET Framework to .NET Core to .NET 5 and now up to .NET 9 is well over a decade long of steady and increasing progress.