Hi and welcome :)
should I always be using a StackedWidget, or are there cases where creating a new designer form class is better?
As often: it depends...
Do you want to have your widgets at the same place and showing only one at a time or do you want them to be next to each other (or in a new window) and showing/using multiple at once?
A QStackedWidget is one of Qt's plenty container widgets... there are also widgets like QTabWidget or different Model/Views.
QStackedWidget works like a book with pages... you can flip the pages in-place, but you can only see the currentWidget.
I didn't want to make it messy and have too many stackedWidgets embedded into each other
Multiple or cascading QStackedWidgets?!
Mh, that already sounds like bad design. Can't see why you need to do this, since you can simply add a new widget to the only QStackedWidget.
There is no need to embed a new QStackedWidget and fill it with content widgets... there might be some edge cases, where it could be viable, but I can't see a reason why you need to do this right now ;-)
Since we know nothing about your app, you should develop a feeling for which design suits your app the best.
Is it more technical? like some simple inputs/outputs, controls and some processing going on in the background.
Or do you have some fancy, modern app in mind, with lots of toolbars, side widgets, menus etc.?
using QT for a personal project.
Because of this, you are not limited and you can try out few things... if you have a more concrete sketch of your app feel free to ask again :)
Happy coding :)