Can I suggest instead of serial popups ...
What about a "wizard" interface? A single dialog that indicates progress through some number of steps? Does GTK have a standard wizard system? (Personally, I don't like wizards dialogs, but I like multiple popups even less.)
No Wizards just [Assistants](https://docs.gtk.org/gtk3/class.Assistant.html), never tried it, don't know how applicable it is.