Starting with human centric morality, a question might be asked, "Should I lie to another person for personal gain?" But to truly answer this objectively, I must first have the answer to the question. "Should I exist at all?" Yet this goes further. until we arrive at a fundamental question of morality that must be answered before anything else can. "Should there be existence at all?" — Philosophim
Lol, I really can't take that seriously though, not only is it non sequitur from sentence 1 to 2 and 2 to 3 but 2 and 3 aren't questions of should. It simply is, so you're asking meaningless questions that beg questions due to missing leaps in logic that even connect sentence 2 and 3 with "should," let alone how they connect with 1. Existence doesn't need to be justified before asking a moral question, or for it to even be meaningful. Morality is a subset of the domain of existence not the other way around.
In your argument morality defines existence because it is so easily reduced to absurdity from the ambiguous definitions that you use Is Ought fallacies to achieve, which we can see because good should be. Saying you don't use the Is-Ought fallacy is like saying Hitler wasn't a Nazi, it's literally in your definitions for all to see, as plain as day as Hitler was a Nazi.
I know I know, you're going to attempt an appeal to emotion via the fallacy of equivocation through taking your definition for adjectival good and substituting it for the noun of a moral good with your example of murdering a child... but that's just another fallacy you use to move the goalpost switching between definitions through equivocation. Sorry mate, I'm not that dumb...
I easily showed how we can reduce your argument to absurdity by the ambiguity of your definitions by line 2 of your OP.
1. Good should be
2. Existence is
3. Morality evaluates Good
4. Existence should be (line 2 of OP)
5. Thus, Existence = Good (cause 1&4) (and the Is-Ought fallacy)
6. Thus, morality evaluates existence (3&5)
7. When in truth you evaluate existence to define morality not the other way round (morality doesn't evaluate existence)
But in your model, since existence is only good (5) all morality is good (because 7 logically morality is a subset of existence), thus even killing under your model is good, as it is also a subset of existence...
Complete utter nonsense.
Furthermore, from your presupposition of objective morality in line 1, we may presuppose the objective morality as:
"Assume the objective morality is that all humans must die, because all humans are mortal"
then its not necessarily that existence should be... making line 2 an occasion sentence.
You should read Quine, and learn a thing about analytics.
Your argument he been a perpetuated farce of fallacy. And you're purposefully dishonest when you swap the adjectival definition of good for the noun of moral good.