Last year, I tried to make a collab tool for 3D through VMC, but I encountered the same issue, and it was the main reason why I didn't continue. I just couldn't find a good way to share the model to all participants.
This is a super cool idea, and I've been working on a similar tool for 9 months. Unfortunately I can not endorse this tool - it is far too insecure!
Operators of Playground get full access to your model, and anyone who gets into your room also gets your full model forever.