Tentative Itinerary: February - March

Instead of revisiting Athens multiple times, stay in Athens at the end of your stay while in Greece. Can you change your ticket from the US to Crete? If not, buy a separate ticket from Athens to Crete.
Since you're renting a car why not visit Thessaloniki then? You're going to and from Athens too many times.

Thessaloniki (3 days, and take 2 days each way to drive, as it's less
expensive than flying)

Since you're talking about cost, in case it's a deciding factor, don't forget that fuel is expensive in Greece (around €1.70/liter / €6.50 per gallon). The cost of driving from Athens to Thessaloniki will be around €60 in gas and €30 for the tolls. You'll have to stop along the way, probably for lunch or drinks, which won't be free.
If you add the price of the rental car itself. In the end, it won't be much cheaper than two €70 flight tickets (and a 45-minute flight instead of a 6-hour drive).

And above all, in Thessaloniki, you won't need a car; it will be unnecessary, you'll probably have to pay for parking (the center of Thessaloniki is a real nightmare for parking).

Other info: The Athens-Thessaloniki train ticket costs €43.

I agree with Marypat that you're going to Athens too many times. Here's a suggestion to optimize your trip:

Start in Crete and from Crete take a flight to Thessaloniki (it's cheap).
From Thessaloniki, you can take the train to Athens.

In Athens, you won't need a car, but you can rent one for as many days as you want for the Peloponnese/mainland area.

Thank you MaryPat and JoLui. I truly appreciate the time you've taken to reply. I hope this doesn't come off as argumentative as that is truly not my intention, I just want to explain why your suggestions don't work for us on this trip.
In answer to your suggestions:
We can't change our ticket and fly directly to Crete. And over the years we've learned that flying from the US in February can be disrupted by weather so it's best to plan on a couple of days in the arrival city (last year we had a mechanical delay of 24 hours and not planning a transfer until 2 days after arrival proved this plan is best for us). Also, Athens is less expensive in February and in February the weather is similar to a cool summer day for us in Alaska.

We do not plan on having a car in Athens.

This plan has Athens in the middle only because of flying in and out of Crete. However, your suggestions have made me consider flying Crete to Thessaloniki, train to Athens where we'd get a car for the Peloponnese road trip.

We prefer a stop where we started at the end of our trip because it gives us a chance to see things we realized we'd missed. And assuming we are going to Dubrovnik it's necessary. We want to fly Aegean and they don't fly Thessaloniki to Dubrovnik until 30 March. If we decide to skip most of Croatia and just do a few days in Zagreb then we might leave from Thessaloniki. In that case we might do a one way car or the train.

Yes, it appears flying to Thessaloniki & back to Athens is cost effective but we will each have a checked bag (small but larger than a carryon) so we won't be buying the least expensive tickets. I've looked at plane tickets for 2 and a car rental (we get a small manual drive) and driving is still more cost effective. And we might stop overnight in both directions to see a bit more of that part of Greece. If we have a car there we know parking can be an issue and would impact our choices for lodging.

Whew! sorry this is so long. Again, I appreciate your responses and as you can see, it gave me a couple of ideas as well

Before choosing between flying and renting a car, wait for Black Friday (next Friday), the day Aegean Airlines usually starts offering flight discounts of up to 70%.

Sign up for their newsletter to receive discount notifications; it's at the bottom of their homepage.

This offer won't last long and is synchronized with Greek time (which I believe is 11 hours ahead of Alaska).

Note that in Thessaloniki, if you are not a resident and your accommodation doesn't offer parking, street parking is very limited. Blue signs indicate parking spaces that can only be used by residents, white signs are for visitors.

If you park on the street you will have to pay via the app: THESi

https://thesi.gr/english/

Three days in Thessaloniki is a good amount of time. Go to the Old Town, "Ano Poli," which wasn't destroyed by the 1917 fire.

That said, the advantage of driving between Athens and Thessaloniki is that it gives you the opportunity to visit, among other places, the Meteora monasteries.

The choice is yours.

Before choosing between flying and renting a car, wait for Black
Friday (next Friday), the day Aegean Airlines usually starts offering
flight discounts of up to 70%.

Sign up for their newsletter to receive discount notifications; it's
at the bottom of their homepage.

This offer won't last long and is synchronized with Greek time (which
I believe is 11 hours ahead of Alaska).

Thanks JoLui. I saw you posted the Black Friday info elsewhere so I'd already signed up for the mileage plan and follow them on Facebook. I just signed up for the newsletter! Apparently Travel Tuesday is a thing, too, so I'll keep an eye out.

Yes Alaska is 11 hours behind, my evening is Greek morning, which means I'll start paying attention Thursday evening so I don't miss special discounts

That said, the advantage of driving between Athens and Thessaloniki is
that it gives you the opportunity to visit, among other places, the
Meteora monasteries

Exactly my thought