Disclaimer: I don't use Windows and use the Prof-Gnome theme on macOS so don't take me seriously.
I'm inclined to having a good user experience by default and the giant tabs of Adwaita aren't a good match for a tab-heavy UI like Geany IMO. This means I would prefer (2).
Regarding what theme to use I must say I don't like the Windows 10 theme much - I can't visually distinguish various UI elements like entries and buttons (try opening Build->Set Build Commands and the entries and buttons are almost impossible to distinguish). And while it tries to use a design language similar to Windows, the result isn't a native-looking interface anyway.
I'm inclined to the Prof-Gnome theme (I'm biased of course) but it could be anything else with smaller tabs where you can tell individual widgets apart like e.g. the Minwaita theme here
https://www.gnome-look.org/p/1174686/
(In this case though I think the Prof-Gnome theme looks more native.)