but certainy there is incredibly valuable knowldge we have now that could still be applied back then

I think it would be highly specific to have useful knowledge that is modern and applicable to the ancients.

Now, there are small things, like better ways to harden metals, or processes that are within reach at the time, but those aren't really modern.

but imagine how much suffering would be avoided if people understood even in a superficial sense about sanitization.

That is a good thought exercise. How would you apply it in comparison to the methods used at the time?

And how would germs be any different from evil spirits in practice to those without modern means and tools?

People knew to separate their waste from water supplies. It was only in more modern eras was that a problem. People knew to cook certain things so they don't get sick. People knew to avoid people who were openly ill.