If your main widget is UUserWidget - then you can use UWidgetTree (WidgetTree variable).
Using UWidgetTree you can iterate over ALL widgets with all nestings.
But there are some nuances with NamedSlot (it seems to me that only widgets that are INSIDE the slots got into the main tree, but the slots themselves are not in the main tree), I don’t remember exactly, but there were definitely problems with it and I had to work with it additionally.