Routes to a Stanford/Harvard MBA other than IBD/PE/MBB?

There are lots of different and well-trodden paths to H/S. Basically anything that puts you into a leadership role and allows you to have impact has the potential to catch their eye. I personally believe that initiative is a tad more important to Stanford GSB, but it's valued by both.

In other words, do something professionally that will allow you to lead and have impact and that you enjoy.

Yes many ways to get there. Professional experience is important. Solid and impressive roles with leading companies are great. Doesn't have to be an LDP gig but those are great too. Companies like P&G, Pepsi, GE, J&J, Microsoft, any of the FAANG, and other leading companies. Rotational programs are great. Would suggest applying after a couple yrs after a 2 yr rotational program so you can show both breadth and depth.

start a company and sell it for $50M+

Work at an NPO in Africa

There are two awesome reports from Fortuna Admissions covering a full analysis of HBS and GSB students from one year. They went through every admit they could find on LinkedIn (90%+ of the class IIRC) and listed out their undergrad school, major, most recent employer, and job title. You'll have to request the reports to see them via this link:

https://fortunaadmissions.com/2019/11/21/fortuna-admissions-harvard-bus…

That should give you a good understanding of where else successful candidates came from, and you can also do a LinkedIn search filtering on the two schools to see the various backgrounds.