Quiz using questions from multiple topics

Ooof. You like giving yourself lots of work Bob! ;)
200 questions?!?!

So everything you describe is easy enough to set up.... until you say "I want to mix the questions from each bank randomly"... and then it gets really fun when you say "...give a total score, and a score for each topic"!

So, first things first - just set up all your 200 questions as normal.
Then set up your Question banks by copying in the relevant questions for each topic.
At the end of this you will have 200 questions (!) in 10 question banks.

If you want to "randomly" pull these at any time from any bank, I think you would be best to use some JS code to construct your entire question array. That way you can set up how many random questions from how many random question banks, and track each question.
Without JS, this is almost impossible I feel.

If you dont want to use JS, then abort the idea of the random draw to "mix the questions from each bank randomly". Instead, you could just set it up quite easily to do this:
Random draw from Bank 3
Random draw from Bank 1
Random draw from Bank 10
Random draw from Bank 6
Random draw from Bank 3
Random draw from Bank 8
... etc etc

It will still be random questions, but you are just giving it a bit of structure. Storyline can do that right out of the box.

Finally, you then need to track your REsults. A single results slide would be easy, but you will have to set up a Results slide for EACH QUESTION BANK.

Then at the end, you will show a COMBINED RESULTS SLIDE which is a special Storyline results slide that takes its scoring not from individual questions, but from other Results slides in your course.

And Im guessing you will need to set up a lot of different layers and output options in your Results slide to cater for all the possibilities that come with having such a huge quiz.

I hope that helps!
But my advice would be to go for the simple options first, get it working with those, and then see how far you want to push it.  Im not sure I would believe that the small gains of the randomisation and extra results is worth the large amount of work it will take to get those... but thats just IMHO :)

Good luck!

Could I hide the individual results slides and only let the user see the combined? So it generates the individual results slides, the quiz bypasses those and the user only sees the combined at the end of the quiz?

Yeah absolutely. You dont have to ever show a Results slide in Storyline. Simply having it will allow Storyline to firstly store all the data relating to your quiz, and secondly, for you to do whatever you want with that data.

So yes, good idea, have a results slide for each question bank (topic) but never show it to the Learner.

When I need to do this with slides, I create a new SCENE and place all my "invisible" slides there. Just helps keeps things ordered and easy to pick up and amend later.

Here is what I did that worked. I decided randomized questions was not totally needed so I have left that but I was able to pretty easily pull from multiple question banks. I started with that. Then added a results slide that scores the combined questions. I added a separate quiz results slide for each of the question banks (topics), then after the first results slide and before the first of the individual results slides I added a Exit Layout slide and disabled the next button effectively keeping them from seeing the individual results slides. Last, I added boxes to the overall results slide for each of the scores from the individual bank results slides. Leaving the user with this...

Seriously good job Bob, well done! I know the amount of work that goes in to something like this, so you've done well.

Glad you got it sorted!