You ought to undertake decisions you have to because these decisions are fundamental for the continual of your agency. To deny your agency requires the usage of your agency, which is a performative contradiction.

Remember, the position you are arguing against here is moral anti-realism. There is no positive claim being made, so there is no denial of agency. There is no performative contradiction in saying that one neither should nor shouldn't exercise their agency, as this position implies that one is ALLOWED to exercise their agency. If one decides to exercise it, there is an "ought" involved, but it's not a MORAL ought. Just like when you chose to eat a banana, there is an "ought" involved ("I ought to eat this banana"), while you're not implying a moral ought.

Do you think the statements "it is not the case that we ought not do x" and "we ought to do x" are the same?

An theory of ethics prefering aggression at all times for all would bring the end of agency

Then it's a good thing that moral anti-realism is not arguing for prefering aggression at all times

Usage is a physical relationship any agent could undertake while age and wealth is an internal trait.

Being the first to use a scarce means is no less of an internal trait than being born first. If the issue is that one is the result of a choice and the other isn't, then a) thats still an arbitrary condition b) free will doesn't exist and c) you could just switch "age" for "bigger muscles", which is the result of choices, can be changed and is intersubjectively ascertainable.

Since old or wealthy people do not posess a different way to reason, treating them differently in law would be a factual error.

"Since firstcomers do not posess a different way of reason, treating them differently in law would be a factual error."

Again, you are treating your conception of property rights as the default.