With the time you have you can see a lot of Greece.
The Peloponnese is a large and wonderful area and just that alone could take your entire five weeks.
However, 2 weeks as suggested would give you a good taste of the Peloponnese.
I haven't spent time in Northern Greece but my research shows that it also is a beautiful area and a different feel from the rest of Greece.
Obviously some time on an island or islands is also a must.
More research is needed and there are plenty of websites and guide books available.
I would recommend the Cadogan Guide for the Peloponnese as it's the best available. While last published in 2010 it's still relevant and filled with more info than any other. Available as a used book on abebooks or ebay.
One of the better websites for all of Greece is:
https://www.greecetravel.com/
I would suggest looking into several websites and guide books to get a more complete view of Greece.
Another good idea for driving in Greece is some type of navigation system which will make getting around much easier. I use Google Maps.
The post hyping Santorini appears more of an advertisement plugging the island rather than a recommendation.