SharpDevelop VS RoslynPad

SharpDevelop is a dead project, https://github.com/icsharpcode/SharpDevelop/issues/799

SharpDevelop and MonoDevelop are both dead and focused for devs which gives them different priority than apps targeted at non-devs, but they show how to put together seriously complex apps, and they're the ancestor of the Visual Studio for Mac.

You should try SharpTreeview. Haven’t had any issues with it for large treeviews

Yes

I've also used this for its WPF editor https://github.com/icsharpcode/SharpDevelop but I dont know if it does WinForm

First - follow instruction in official Git account https://github.com/roslynpad/roslynpad:

Sometimes i use roslynpad to check some code sentences quickly at my work. I think u could use it for practise, its a really simple C# editor. https://roslynpad.net/

RoslynPad is similar but not quite as good. https://roslynpad.net/

RoslynPad:

if you clone roslynpad (https://github.com/aelij/RoslynPad) you can find samples. It already deploys with Roslyn (the runtime C# compiler) which helps a bit with integration.

There is RoslynPad but you have to build yourself on Mac OS.