I have now created lots of question banks and about 3000 questions. It took me a long time to become efficient and work around the limitations of Canvas's editors. So I thought it might be useful to post here some of the solutions and save someone else some time.
1) Use an image hosting service. If you need to use any images in your question banks, be sure to use an image hosting service (I use Amazon S3). Using any other sort of links break down when importing or copying banks from course to course. If you have thousands of questions, you will have to manually edit each question again to link to the correct image.
2) Manually edit your banks. Question banks are just xml files, so they are easy to edit in any standard text editor. If you learn a little grep searching, you can create banks of hundreds of questions in a fraction of the time it would take to create them inside of Canvas. This works really well with banks of similar questions. I cut about 8 hours from the process of creating a 1600-question bank just by using find/replace.
3) If you are starting from scratch, create one question in a bank Canvas and make sure it looks exactly the way you want it to look. Then export the course (This is easiest to do in a course that contains nothing else). Change the file type to .zip and unzip it. Look for the folder called "non_cc_assessments". Inside that folder should be your question bank file, though it will be named a long ID number ending with qti. Copy that file to where you want to work (You can rename it whatever you want). I set my computer to automatically open qti files in my text editor.
4) Edit your file. You'll see the bank info at the top of the xml file, and each question begins with <item> and ends with </item>. If you want to create a NEW bank, change the name of the title near the top. If you want to copy your question, just select everything inclusive of those two tags and copy. Then edit each question as necessary. The answers to each question come near the end of each item. Options have ID's which you can change to whatever you want, just make sure the correct answer matches the correct option ID.
5) After creating your bank, be sure to zip the file and upload to Canvas.
Hope this helps someone!