EDIT:
I've found where this variable is set (trying just in case, but no luck):
GTK_MODULES=canberra-gtk-module:appmenu-gtk-module
(to remove appmenu-gtk-module)
It's in
/etc/X11/xinit/xinitrc.d/80-appmenu-gtk-module
So I edited it to skip all of it in case there's the NOAPPMENU set, which I set in the
~guest/.config/environment.d/custom.conf
Now GTK_MODULES is without appmenu-gtk-module, but still no menu in dolphin
Edit: what worked for Dolphin: Dolphinrc file reverted by dolphin at startup - but I need to set my preferences - #10 by Dadrake