I find nothing in Genesis to conclude each of the 6 creative days were one-thousand-year long days.Exactly. This is why we should not dismiss science saying the earth is 4.5 billion years old, when our guesses and calculations using the Bible only tell us it's 6000 years old.
There is nothing there to even indicate that each creative day was of the same or of differing lengths of time.
For that matter, each of the creative days are summed up by the single world ' day ' at Genesis 2:4.
So, nothing in Genesis is out of harmony with CMBR about earth's age.
Plus, Genesis is telling us more about getting Earth ready for mankind to inhabit Earth.
God's 7th Day is still on going as found at Hebrews 4:4-11.
Interesting that you point out that there are some people who say Earth is only 6,000 years old.
I've heard it said there are people who believe God only made the young Earth look old.
Scripturally that would be wrong because that would be saying that God is deceitful.
- God can not lie / deceive -> Titus 1:2; Hebrews 6:18; Numbers 23:19