i solved some time ago, (i dont remmember how i did it) but it is possible
if you decide to use app inventor i could search again and tell you how i did it, at this moment i am opening the app and when i hit the back button y programed a message that ask you if you whant to exit, yes or no?
so… there should be somewhere a programable behaviour to the back button.
but… as you said… probably app inventor or thunkable are not solid as android studio… if you can program in android studio go for it (and share :D, if you place the www address in a variable that anyone even with no android studio knowledge could change so we can use your app would be grate)