I've got a similar problem with trying to copy a quiz. We've been using Moodle for around 10 years now and it's to the point where we try to either import an existing quiz to a new course or try to back up the course and then restore it into a new course, it fails... period! For years we would use the import method where we create a new course and then import the quizzes from a previous course and it worked fine. We then learned that doing a backup on the course and then restore it was a better way, so we've been doing that for a couple to three years now.
Luckily a couple of years ago we created a file for all of our backed up courses so we only have to select the course and restore it, which at this point is still working but it takes a while to restore. This works as long as long as we don't change a course so you can see where that's going. We have one course we rebuilt (and others that I would like to) and unfortunately we have to build every quiz brand new each time we run that course because we can't backup nor import a newly developed course.
As always, everyone's circumstances is unique and solutions tend to vary. For us, Every question I have is categorized in the question bank under the "Top" section because as I see it, I want that question to exist one time and that makes inventory control, maintenance and tweaking questions easy to do. I've had it beat into me by database
programmers that information should only exist in one place and then you use it as needed, so that's the way I'm wired. This mindset doesn't work well with the way Moodle copies from one course to the other because as you say, the entire question bank (in my case) has to be copied each time you build a new course from an old one.
What I'm going to try is to carve out a few sections
of questions that for the most part belong to a unique topic and are not generally used throughout our system, and move them to either "categories" or "courses" so when we backup/restore or import that it's not copying as much data. What I don't know is will this be retroactive and thin-out what's been building over the years that has me bound up at this point. I've also thought about taking a few years of history and archiving them somehow but I have no idea how that would be accomplished and still have access to that history. Past that, if no one has a solution, I'll have to start a brand new Moodle and find a program like Respondus 4.0 to move my existing questions from the old Moodle to the new one without transferring the clutter data that's got my system bogged down.
I'm in no way trying to hi-jack your post. However, we do share the same problem with different circumstances and maybe some of the things that I have tried or thinking about trying may help.