For overpopulation, Mechai Viravaidya's work in Thailand on lowering birth rates, plus many peers, like Costa Rica, Iran, and more. I posted a bunch of podcast episodes and blog entries on him https://joshuaspodek.com/?s=viravaidya, and YouTube has more.
On overconsumption, I think of Bea Johnson https://joshuaspodek.com/guests/bea-johnson and Josh Becker https://joshuaspodek.com/guests/joshua-becker, just among people I know, improving life on individual basis, then systems people like W. Edwards Deming who showed systemic change begins with personal transformation. He worked in other areas, but led to national and global change on the scale we need in years.