I can’t help but I want to say it is good that you have a writer. Start with a story. A writer can produce a real story and not just ideas. Even a good draft communicates the theme, style, tone, and main features to your team. Your artist’s visualizations will further strengthen that foundation.
I tried to get a team together like this once. Similar situation. Needed a team to do it. For fun but hoped to become more. I had a theme and ideas for a world and stories. People I know IRL or from CG forums were interested. Pro voice actors, pro video editor, amateur animator, skilled CG hobbyists, science advisor, veterinarian for creature design, amateur screenwriter. The main thing that stopped that was I only had ideas. All good people but just ideas was not enough to define the project and motivate them. Might try again someday but only if armed with at least most of a story and better developed theme.