Why not one, big menu with everything on it? Catagorized of course, and even 'jump to section' buttons, but you can still see everything on one page.
... or because certain ways of expecting a GUI to work have become almost hardwired into us. That's what I'm saying.
1. It probably wouldn't fit on computers with small screens, all told Geany has a lot of UI when you include data entrys which is what dialogs are for 2. its not supported by the GUI toolkit 3, as you say, its unfamiliar to users, making switching applications more difficult, very few people use only one app
You're in luck. Geany has such a text file. On my system (Kubuntu 22.04 LTS), it's the hidden ~/.config/geany/geany.conf file.
Editing this by hand is a bit tricky, Geany only reads it on startup, but Geany will write it on close, overwriting any edits, so you need to have Geany closed while editing it, so you need a different editor. Probably best not to hand edit it.
Cheers Lex