Small update here:

I have now ported all things that was clear for me to gtk3

The other things now seems to be quiet difficult to port now and I have not found a good porting guide for C#

I only found this things:

https://docs.gtk.org/gtk3/migrating-2to3.html
and
https://docs.gtk.org/gtk3/migrating-themes.html

Maybe someone have a clue how to port the other things.

We also have a lot of protected override void Render in the code. How to replace in gtk3, when trying to enable it says:

  /home/maki/Desktop/dotdevelop/gtk3/main/src/core/MonoDevelop.Ide/MonoDevelop.Components/CellRendererImage.cs(133,27): error CS0115: 'CellRendererImage.Render(Widget, Rectangle, Rectangle, Rectangle, CellRendererState)': no suitable method found to override [/home/maki/Desktop/dotdevelop/gtk3/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj]

    214 Warning(s)
    1 Error(s)

I'll see what I can do with migrating the entire project to GirCore (GTK4) instead.