Regarding gtk3-mashrooms, perhaps it is better to talk with Canonical devs to carry those as patches. Specially no_csd patch. It can be done for specific sessions (like mate, xfce) may be tied to a gsettings key. Or may be you can reach even debian. They already carry few patches.
Scrolling on notebook would be a nice addition too. Since gtk3 is stable now, it can be maintained easily.
Alternate way is to use gtk3-nocsd which is not maintained anymore or perhaps packaging it as a gtk-module (note, gtk-module was removed from gtk4).