> Which in turn depends on the endosymbiosis of mitochondria.

Which in turn depends on existing enough atmospheric oxygen for mitochondria to make sense in first place. I believe the filter is atmospheric oxygen. If Earth had more iron in the crust, perhaps the cyanobacteria would never finish oxidizing all of it, and we would be doomed to only host microscopic life forever. Macroscopic life requires high-energy metabolism molecular oxygen allows.

Could that be down to planet size?

We have lots of iron , but most of it sank to the core during the "iron catastrophe" . And Mars is red because of theres so much iron.