What is the usage of 物 here?
I think that "the thing" can't be explianted precisely. We can't give it a name because"名可名，非常名".
When it comes to the relationship between "混成" and"物".It's naturally to say  that "混成" is the adverbial of "物".
"混成" have several meanings at the same time.
1： 混杂而成：Everything is contained in "the thing".
2： 混然一体：However,"the thing" can't be separated as it makes no sense—— just like "ok" is not a part of "book" although "o" and "k" is 2 letters of the word "book".
3： 自然形成："自然" is more suitable to be explained as"by itself" than "naturally" here. The orign and reason of "the thing" is itself so it's a perfect thing.