I ran across a theory before that tried to connect the Hellas Planitia impact basin, Valles Marineris, the different of crust in areas, the lack of geologic activity now (dead core), and the orbit to suggest a large body impacting long ago. It seemed to make sense generally, we know it happened to Earth to create the Moon. I don’t know if it holds up to harder analysis or what evidence we have now. But it basically said that Mars got hit by something closer to its size, much larger than Theia for Earth, that left a huge impact zone, killed the inner part of the planet, and split the other side in some places while blowing off crust and atmosphere. It also shifted the orbit away from a “normal” one like the other inner planets. It was a bad day on Mars. If true. But I do think something happened, there is scarring, even if it wasn’t exactly all that.