Hello Brett,
I have not been able to get Marked 3 to work with pandoc as a custom processor (I already copied pandoc to the Marked package). Since the first release, when I open a large file (~8000 words), Marked just stays on the spinning M (nice animation, btw). This morning, after the latest Setapp update (v. 3.0.9), I was able to properly render a smaller 800-word file. However, after working on it, Marked crashed, and now I only see this error message:

I've been using Marked since v1 and it's become crucial for my workflow.
Thanks a lot!