Are you talking about omicron now or original Wuhan virus?
Omicron stays mainly in upper respiratory parts. It does not go deep into lungs and body as other variants. That makes it less lethal, not because it is somehow milder. Normal cold is also quite lethal, once it spreads through body, but it never gets a chance.
I am happy to accept theory it stayed in single host for several months, and gained a few mutations. But that does not explain its DNA code. You do not get this type of mutations without spread in massive population. And there are no random bit flips, that should accumulate over several months.
> But that will prove exactly nothing to anybody who believes in lab leak origin.
This is not a religion, there is no "believe". Lab leak (or artificial synthesis) is just a theory that explains origin of virus very well. Dismissing it without investigation would be unscientific.