This didn't happen. There was no selection pressure on the virus to mutate to a "less aggressive" form. To think there was is to fundamentally misunderstand the science here.
The incubation period was plenty long enough for the virus to spread before incapacitating the host. All the selection pressure was for the virus to become more virulent - and that is precisely what happened. We saw multiple strains appear which were harder to deal with.
> Reducing counter-productive treatments (like ventilators) helped greatly too.
This had a negligible impact. Patients were only put on ventilation when they were already very sick and at a high chance of death. Worldwide only a tiny proportion of deaths came about in this way. Even rich countries only had ventilators in the tens of thousands. Compare that to the billions who received vaccinations.