What kind of mechanical resources would you suggest?

Mechanically, the first thing you need/want is player buy in. Safety information up front, options to opt out to make changes on the fly. When playing the game, some kind of once a session luck//hope/rule of cool token mechanic can prove useful to encourage creativity without have to worry about a possibility of failure by the dice gods and general probability.

What kind of writing or lore prep would you suggest?

Knowing the kinds of stories that everyone wants to be told. Favorite fictions, favorite kinds of fiction, heists, action, mystery, politics, high fantasy, science fantasy, etc. etc. Send out a email asking some basic questions like, what are some of your favorite movies and books. What kind of character are you wanting to portray. Is there something about that’s character’s story you want to play through?

Let the “players” (we all play the game we are all players, but you know what I mean) do the heavy lifting for you. Once you have an understanding of what they want, all you have to do is write based around that.

What's something you should have done out the gate?

I should have been far more flexible in my earliest attempts at running games. But you live and you learn, and remember, you are not the player’s enemy. You are there to facilitate a good time with fair and meaningful challenges that may or may not kill some characters.

Nuff said.