Unfortunately my best recommendation is reddit.com/r/AcademicBiblical - though I'm hoping to eventually pull that community over to Lemmy.

It's a pretty neat place where you have a range of bright minds from various degrees of experience discussing a lot of topics like this.

In terms of the topic of proto-Mark you can see Burkett's book on the subject.

And then for the relationship between Paul and the gospel of Mark there's the aforementioned book.

But one of the challenges with the book route is that you typically get a lot of a single perspective, whereas in communities debating the books you can quickly get to some of the interesting overlaps or differences between perspectives.

That sub is probably the only thing I really miss from Reddit.