My version of Genesis doesn’t mention DNA at all, so if God used different variations on the theme when creating all the animals, it wouldn’t in any way negate what was written in Scripture.
On the other hand, if you put millions of years in between night and day, it is likely the plants created on one day would be long gone before you got around to creating the animals.
Of course, God could keep them alive through the long cold night as well.
The difficulty (and it is not insurmountable) to abandoning the idea of a 6-day creation is that there is a lot of religious observance, including that taught by Jesus himself, which is tied to that 7-day period. There’s even a commandment which was given directly by God, which would seem odd if the entire 7-day thing was just allegorical to begin with.
I would be more likely to believe that there were living things throughout the earth, but that “creation” describes a specific act related to the Garden. Except that doesn’t really work well either.
It’s simpler if you realise God can create things with the appearance of age. This isn’t some artificial construct — Adam was created as a fully formed man, not as a embryo, so Adam had the appearance of age.
You go speculating about the nature of his birthing you open the door for everything else.
Chapter 2 of Genesis doesn't contradict chapter one, it just fills in some detail about certain events.