I'm looking for guidance that is focused specifically towards those who are managing managers for the first time - in other words, first-time directors.
I've been promoted to a new role where I'm now managing managers rather than individual contributors. The managers who report to me are first-time people managers and I'm keen to set them up for success.
I'm planning to roll out the MT trinity to the team. Beyond that, any recommendations would be much appreciated.
This podcast has been really helpful: https://www.manager-tools.com/2010/01/developing-subordinate-managers-chapter-1

Delegate for Learning
I like delegating tasks into people's weaknesses. For example, I have a manager who is in a niche area and doesn't have a large network. So I made him my hiring lead to coordinate all hiring across my entire team. Plus he has to report metrics up to the Directors and VPs on occasion. Another was having some issues with collaborating with his peers so I made him my cross-training lead. Of course, sometimes the right answer is to delegate into their strengths as well.