"I've struggled to find a good argument against suicide ...."

Your OP lists some kinds of answers you would consider *not* acceptable, but it is not clear to me what kind of answers you *would* accept.

You received many responses, most within the framework of hedonistic materialism, and you've rejected all of those, maybe with good reason. By hedonistic materialism, I mean: pleasure is good, pain is bad, nothing else is good or bad except as leading to pleasure or pain; there is no God, no immortal soul: when we die, we cease to exist. Is that the framework you also are operating under? Would you be open to answers that question or deviate from those ethical premises---for example, Kantian or other deontologies, virtue ethics, natural law, non-hedonistic consequentialism? Or questioning the materialistic assumptions? It seems you've poo-pooed metaphysics, but it is important to realize that materialism, too, is a metaphysic.

Or maybe you are not looking for an ethical answer at all, but only considerations of "self-interest":
egoistic, what some would call "prudential" reasons? Do you think there is an objective rightness or wrongness about suicide, and if there is, do you care?

"How dare you judge them." People who judge that suicide is wrong are judging a kind of act. They are not necessarily judging any *person*. You yourself, I hope, will agree that judging an act is morally okay, because in that very sentence you were judging an act of judging.

"they were the most moral person I ever knew."

Seems to me this is begging the question. "They" may be the most moral person you ever knew *except* (possibly) in the matter of suicide. So then *if* suicide is okay, then yes, they were very good. But if suicide is horribly wrong and they did it, then they also did something horribly wrong. Same as a man who is morally upright in every way except that he rapes young girls. I won't deny that your person was very conscientious and did what they *thought* was right, but so do people who commit horrible crimes against humanity for the sake of some twisted political ideology.

"[Who owns a life?]. The owner. This seems to essentially mean between ages of about 0-16, the parents of that child (or, their caretaker/s. We seem to legally agree with this position). After that, it is the person who is living the life in question. They are free to do as they please with their life (hint: Not other's lives, which will come into play for 3.);"

Yet parents do not "own" their children in the same way they own material things. If I own a book, I may cut it up, beat it without mercy, burn it, sell it to another. I may not do that with my child.

If I may not kill the child whose life I "own", it's not clear why "owning" my "own" life as an adult would mean I may kill myself.

Perhaps it is not safe to assume that any of us owns *any* life, even the one we call our "own."

People who judge that suicide is wrong are judging a kind of act. They are not necessarily judging any *person*. — Gregory of the Beard of Ockham

Good point - separating the person from the behavior

"They" may be the most moral person you ever knew *except* (possibly) in the matter of suicide. — Gregory of the Beard of Ockham

And this begs another question - in what circumstances is suicide moral?

I few posts upthread I shared my personal experience with my spouse, and I am very satisfied with the morality of his decision to use MAID

And this begs another question - in what circumstances is suicide moral?

It's moral if the individual is competent, free from external coercion and dealing with permanent agony/suffering.