Great Idea! Totally go for it as a Hobby. It'll be fun... just use Microsoft Visual Studio and create a windows template so it does the layout for you. It's faster and keeps you focused on the important stuff. If you like you can learn how to make your own Windows for the UI.
Yes! This is what I was thinking too.
Ms vs is quite good for programming stuff and there err also compilers for Linux with c# if I manage to make anything useful. I think there are also built in things for visual studio too. I’ll have to try to spend a month designing and looking at how bitcoin core functions in order to write my code to be as compatible as possible. (It needs to be able to down,lad and validate the blockchain for a start)...
if you want your code to run on multiple platforms and not just be for windows then you have to write it using .Net core and avoid anything that is windows focused (e.g. WPF is only for windows, .Net framework is only for windows,...).
my suggestion is to use .Net standard library project type for your libraries and for your GUI you can use Avalonia[1] which is a WPF like GUI that works on multiple platforms.
and good luck.
https://github.com/AvaloniaUI/Avalonia