Keep it short unless you want to achieve something specific, and then include only the verbiage relevant to that purpose.
I have a better offer for a job I'm looking forward to starting, so there's nothing I can do that will help me personally. It feels more honest to me to be upfront and, who knows, it might help my colleagues.
I don't think being frank will affect my ability to get references from the people here. Aside from that, are there any other risks?
That's a mistake. You may see it as frank and constructive feedback, but they may see it as complaining and acting unprofessional. Is it really worth risking it?
Keep the formal resignation letter short. It's perfectly fine to include a few pleasantries (unless you plan to sue them for something in the future.)
Then, if you wish, have some private conversations with management and give them an honest assessment of how they could better achieve their own purposes. Keep the focus on helping them. Take your former bosses out for coffee they next time they visit. Give them insights that they would not other get. Keep up your relationships with your former colleagues as well.
That's the purpose you should keep in mind. It's more than not burning bridges. It's about creating future opportunities for yourself. Create lemonade from today's lemons.
Edit: do this even if they're all evil bastards. You never know what will come of it in the future.
If you really want to let them know why, tell them in the exit interview.
If they don't perform an exit interview, well that tells you they give no fucks about what you think anyways, so you'd be wasting your breathe.