What strategies have you found effective in creating lean organizational structures with minimal layering,  while ensuring appropriate experience levels at each layer? What guardrails or criteria do you use to ensure sure you're not promoting talent into a role that's too big/advanced?

Christal clear job descriptions, same for role expectations within the organisation (how does that role contribute to the business success? This needs a clear answer), then as others already wrote too: clear criteria of how someone from a level below or horizontally from another role can successfully step into that role above/nearby.
If these are clear for everyone in the organisation and it is adhered to (that's also important, adherence), then you can get there.

- Critical to have clear job descriptions available as well as Calender oF Discipline (what is expected of people in a role as well as understand what activities are done daily/ weekly etc
- For promotions, suggest to rate criteria against competencies shown in previous projects, ambition (does the person want the role) as well as giving the talent the right tools to be successful (additional training/ coaching, mentoring etc)

A few things come to mind. As far as a lean and flatter org structure you typically have to get comfortable with your leaders covering broader scope including areas where they may not have deep subject matter expertise. In past roles I’ve done a couple things to help here.. 1) create the concept of program leadership with technical leads where they have more responsibility and decision rights, but may have no one reporting to them 2) use spans and layers to keep myself honest about how flat the org should be.
On point 2. Having Job Leveling defined helps people prepare for that next step. Most of the time it seems someone ready for promotion is already taking an informal leadership role and performing at the next level fairly consistently. Second, I’d say I’ve been too slow to promote people more often they over promoting. Most people that have left my teams get a one- or two-level bump at their next gig. It seems the market knew they were ready before I did.