Thanks for sharing your profile.
Would have been great to get the outright admission but you are certainly still in the game, schools certainly pull qualified candidates off the waitlist all the time. I don't think taking additional courses or the GMAT again are real profile movers for you so you just may need to be patient.
As far as next steps go I would:
First carefully review the waitlist notification, not sure how those schools are doing it this year but if it is an opt-in make sure you let them know that you would like to remain on the wait-list. I believe they usually ask you to wait to reach out for a few weeks before you connect with your Waitlist Manager and you should follow these directions. Once you get a hold of your waitlist manager, I would ask for an application review, they may not do this but the goal really here is to get some insight into where there were reservations about your application.
Once you identify those areas, I would next decide if there is an opportunity to "significantly" improve that area. If there is then you should figure out how to do this ASAP and send a small update package. In the meantime if you don't want to wait you can start pulling together some supporting documents.
If applicable, these supporting documents will serve as your updates. I think "substantial" updates can't hurt. Focus on material updates that address potential weaknesses and/or showcase positive aspects of your career trajectory. Keep it positive, many people were denied admission outright you are still in the game.
Hope this helps!
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