Retain?  Where was he going to go with 4 years left on his contract and nobody knocking on his door to coach at their school?
I'm more in line with 
@MADHAT1 's approach.  See how he does next season, but offer him performance incentives in advance for the 2024 season.
Extending him out 7 more years from this season after 6-6 is over the top.
This is not saying I don't want him to stay.  But the rules of negotiation are what is the other party's next best alternative.  What was Greg's next best alternative currently?