Nothing in what you said before: "It is it's [sic] own explanation because literally nothing else can be and without it there can be no contingency.", actually specifies that it cannot be 'a composite', nor does it say anything about complexity or change (not that the whole space-time and its contents actually does change).
Basically you have never explained how anything can logically be its own explanation, you just keep on making up bullshit about it...
It seems from what you’ve been saying that by reason you mean external reason. The onus is therefore on you to explain why only external reasons are valid.
However you also seem to be hedging in order for the universe to exist without needing an external reason.
The two positions are contradictory
We know that things exist. You suggest, in one interpretation that the universe is a single entity in which case it may exist for the internal reason that there is nothing external to dictate it’s existence or lack of it thereoff.
The trouble is the opposite is observed the universe is not a single entity and there are entities whose existence is dictated to by external reasons. So we are left with two questions what is responsible for contingency? And why everything must have an external reason.
So, have the universe as a single entity and consider whether logic, science, and reason can work in it.