Hey ecrypt, this will sound like a real cop out but, do you know you can make Qt5 apps in a GTK2 based IDE ? You do not have to be using a qt5 IDE. Don't think I am saying the problem you report here is insignificant, its not !
I recommend you do make you app in qt5 but use a gtk2 version of lazarus IDE to do so ! In fact, in most cases, you will find its trivial to make you app recompile using gtk2, qt5, qt6 ....
Davo