> The earliest strains of Covid were by far the most severe ones. It's not that surprising to me that the very first patients contracted a strain more likely to put them in the hospital.
No, the only SARS-Cov-2 strain with (likely) decreased mortality is Omicron, see https://en.wikipedia.org/wiki/Variants_of_SARS-CoV-2 The fact that Covid (as an illness) has become milder is due to the hosts adapting, not the virus being milder.
> The severity also depends a lot on viral load.
Yes, that is a factor.