How does this itinerary sound? (Santorini, Athens, Delphi, Olympia)

Day 8 is exhausting and rather difficult, almost impossible. Napflion is the center of the Mycenae and a visit to Epidavros should be included. Napflion is one of the most most beautiful small cities in Greece so give it enough time.
It is quite a drive to get to Olympia. This area is mountainous with twisting roads. We were so glad we visited Olympia so try to add at least a day here. Staying overnight in Delphi with it’s incredible views out to the sea is recommended.
As an aside, Santorini will not give you a real Greek experience. Perhaps with COVID all the workers from other countries will not be there changing the culture.. We dealt with non- Greeks while there and it was so over touristed. Paros or Naxos would give you a better Island experience.If you do go to Santorini a must see is Akrotiri, the ancient Minoan site. We enjoyed being down on Amoudi Harbor below Oia for lunch.

Agree with Suki regarding Day 8. I'd be tempted to stay in the Nafplion area for my last 3 days and skip Olympia and Dephi - or at least one of the two. FYI - I've only been to the Athens area and Santorini. I did plan two driving trips on the Peloponnese that were cancelled due to a transportation strike and COVID respectively.

In the Nafplion area --- in fact, just about 5 miles off the big intercity outbound highway -- is another "Sacred Games Site" that will give you the feeling of the ancient Games without that day-long drive ... It's NEMEA. Beautifully excavated, authentically restored (U of Cal Berkeley has the $$ to do it rigt) ... it has ruins, temple columns, and an AMAZING stadium, with the original "heroes' tunnel" for entry and racer's starting-blocks from 400 BC. Good Museum too. What it doesn't have? Crowds... u are almost alone. Stop on yr outbound drive to Nafplio... & spend that extra night in Greece's most beautiful Old Town.

Nothing wrong with KBK’s recommendation at all and it would be a lot more relaxing. But if you really want to visit Olympia, I might arrange to fly from Santorini back to Athens in the morning on Day 5 and drive to Nafplion immediately for overnight. Day 6 drive to Olympia with a stop at Mycenae. Day 7 tour Olympia. Day 8 drive to Delphi for overnight. Day 9 drive to Athens and return car and see Athens. That is a LOT of tiring one night stops, but you are looking to cover a lot of ground.

Otherwise drop Olympia and stay nights 5 & 6 in Nafplion, night 7 in/near Delphi, then nights 8-10 in Athens. Suki is right that Day 8 would be really tough. If you had an extra night, it would be nice, but I know that may not be possible.

I am in agreement with others who say day 8 is too long.

I might suggest you re arrange a few things in order to get the best logistical plan.
Arrive Athens immediately fly to Santorini. Perfect. waking up on an island is the best way to get over jet lag.

Day 5 return to Athens. Many experienced travellers suggest that you hold off on visiting Athens to the end of the trip. So since you are probably going to fly to Athens rent the car from the airport on day 5.

Drive to Nafplio. On the way stop at Korinth and visit the amazing sites there. The canal, Ancient Corinth, The fortress of Acrocorinth and Ancient Nemia mentioned by Janet. Nemia had sister games to Olympia. I am suggesting you drop Olympia and save the two days and eliminates 10 hours of driving to and from Olympia.
Corinth Area. https://www.flickr.com/photos/stanbr54/albums/72157711755443483

Make Nafplio your base to explore all the ancient sites nearby. This would be day 5,6,7
Nafplio https://www.flickr.com/photos/stanbr54/albums/72157632094108982

Delphi on day 8.
Delphi and Meteora https://www.flickr.com/photos/stanbr54/albums/72157698930270992

Athens day 9 10 and depart day 11.
This is way less driving and allows you to explore amazing sites without being rushed.

Echoing the comments about Olympia being a long drive. The archaeological site is okay. My trip would have been fine without a visit there. The museum is quite nice. The town is ok - very touristy. If you stay overnight (we did not), I'd look for something out of the town.