Re: To a first approximation
To a first approximation, if you think that first approximations should lead to a true statement then your thinking is flawed.
All scientific understanding is an approximation. Sometimes a very good one. Sometimes one that does not apply in rare circumstances. Newtonian gravity and mechanics is a very good approximation that fails in some circumstances. If you're designing particle accelerators or navigation satellites then you need Einsteinian relativity, otherwise Newton does just fine. Even with theories which give very good results and have no known exceptions we cannot prove (or even know) that they're true because new evidence or thinking could cause them to be supplanted.
In the case of first-order approximations all we ask is that they provide answers that are close enough to be useful. Often all they have to do is let you decide if something might be feasible (so try a more accurate approximation) or clearly impossible (so no need to do all the extra calculations).
You're right that a lot of the elements important to us are not metals. That doesn't alter the fact that most of the universe is hydrogen and helium. From periodictable.com's abundances we get hydrogen 75%, helium 23%. All the others add up to 2%. So, to a first approximation, the universe is hydrogen and helium.
Excluding hydrogen and helium, there are 15 non-metals in the periodic table and 7 metalloids. If you class metalloids as non-metals that means 70 out of 92 elements are metals. Class metalloids as metals and that means 77 out of 92 elements are metals. English doesn't have a word meaning "every element but hydrogen and helium" so when you're looking at stellar spectra to see what's in stars, after you exclude hydrogen and helium lines then most of the lines you see are due to metals. This is expecially true because elements with high atomic numbers are metals and have large numbers of transition levels. What you're going to see in a star's spectrum is mostly hydrogen, helium and metals.
It's only when you look at the second-order approximation that things change. Oxygen, carbon and neon are most abundant after helium. Iron after them. Then silicon (metalloid) and magnesium. Then sulfur and argon. Etc.
And then you get to the third-order approximation. During planet formation, a lot of the lighter gases were driven away from the inner accretion disc by the solar wind, so our planet is has a massive iron (relative abundance in universe 0.11%) and nickel (0.006%) core.
To a first approximation, the universe is hydrogen, helium and metals. Especially if you spend a lot of time studying stellar spectra. A geologist would give you a different answer for planet earth.