Installing Windows first is just the recommended way to set up dual-boot, since the Windows installer will always overwrite the bootloader without asking any questions. I think these days there's an automated repair tool on the Ubuntu install media that'll take care of fixing it, though. And with Nvidia drivers it's usually a good idea to install the latest ones from the "graphics-drivers" repository (there should be plenty of guides for that too).

Also, depending on what kind of C# development you do, VS Code can be very nice. DotNET Core is very straightforward to work with, and targeting the full frameworks with Mono is doable with a little bit of tweaking. If it's Unity, then the debugger plugin will be completely useless on Linux, and you'll have to use the very latest beta version of the editor.