It also implies that most normal work isn't in the interest of society, which if that were true, would be a major problem on its own. In what sense is 2 years of military service or "community service" strictly better for society than going to school, or working as a waiter, or starting a small business?

Roughly speaking, 2 years of community service should be worth some % of local & federal taxes.

In a developed society, I'm not sure what kind of labor an 18-year old can perform (what I mean is that it would be mostly unskilled labor), that would be better than taxing this same individual later in life, without delaying their education by 2 years.

I suppose there would have to be exemptions for college students as there typically are in such schemes in other countries?

I mean, yes? I think when we say, we should force 18 year olds to do something useful for society instead of working doordash or taking college classes that they don't care about, that is kind of saying something about the usefulness of those things.

If the market figured it out we wouldn't be having these discussions in the first place.

The only reason to force two years of service is to reinforce conformity and suppress outward signs of neurodivergence.