apt install qt5ct adwaita-qt

QT5 Settings -->
A) Appearance
1) style adwaita
2) standar dialogs GTK3

B) Fonts
same as apperance (xfce mate etc)

C) slow qt apps?

sudo apt install appmenu-registrar vala-panel-appmenu-common

With those both installed (and the recommends from appmenu-registrar), after reboot your qt5 apps should launch without delay,