Car Rental for 2+ week road trip in the Peloponnese

We rented our car from AutoEurope at the Athens airport and it was so easy to drive to the autostrada and on to Nafplion from there. We always rent our cars from AutoEurope throughout Europe and have done this for more than 20 yrs..

We had excellent service from Athens car rental. Worth the extra $30 for delivery to our hotel. Service was on time and friendly.
Maps. We used Google and maps.me.
We bought a sim card on the street. There are many such vendors with tables near the tourist area. Like the Acropolis area. Cheap too. We got 30 days cards. Helpful and they activated them for us. We got two since they were so cheap and thus we each always had web access. Access was all Europe so after Greece we were in Bulgaria and the sim worked there.
We drove Peloponnese as you are. Easy. Lots of two lane roads. Not much traffic. Usually takes longer than maps app says. We paid for the toll roads as needed as it was a relief to be driving faster and easier than the small roads which are fine but you get tired of the slow pace.
Have a good trip.

It's not a bad idea to go to Koropi metro station to pick up the car.

There is no ACR office at the metro station but I assume they will tell you where to find the car.
Maybe the ACR guy will wait for you on the bridge where the metro exit is located. The bridge passes over the highway which is precisely the one that goes to Nafplio.
But it would be just as easy to go to the airport which is just one metro station away.

Driving in Greece is not difficult. To go from Athens to Nafplio it will be 80% on motorways.
The travel times indicated by the GPS are more or less reliable in this part.

Regarding the rest of your itinerary (Mani, Olympia, etc.) these are semi-mountainous regions with often winding roads and lots of places that make you want to stop.
I always add about 15% of travel time compared to that indicated by the navigation systems. And I also add more or less 1 hour because as a human I have needs such as eating, drinking and enjoying the places where I am rather than sitting for hours contemplating my steering wheel :))

We have rented twice at Athens airport (Enterprise & Sixt) to drive to the Peloponnese. The toll road/highway is very good, well signed with frequent rest areas AND very easy to bypass Athens town. Other roads are marginal to good with erratic signage and often hilly and windy. Add time for conditions and view stops. Having the car delivered & picked up at your hotel is good except, staying in town (near Syntagma Square?) means having to drive out and back through traffic plus finding parking. Bus X95 at Syntagma Sq is a cheap way to/from the airport. There are other airport buses in different areas around Athens. You really do not want a car in Athens unless you have nerves of steel and a private parking spot.

I always get a paper map (Michelin) if only for route planning . In any case, use an gps app with offline preloaded maps as cell coverage is spotty. Enjoy your trip - the Peloponnese is great.

I can't help with renting a car in Athens.

I normally take the bus from Athens to Nafplio where I pick up a rental car at Staikos Car Rental which has the best prices in Nafplio.

As far as driving in the Peloponnese I use Google Maps for navigation and for the most part works well regardless of the terrain.

Paper maps are useless for the Peloponnese as they can be cumbersome and hard to figure out.

The roads in the Peloponnese are good but can be slow driving especially due to the terrain, mountains, curving coast lines, driving through villages, etc. Don't expect the times from Point A to Point B given by various navigation systems to be correct as it can be much slower than expected.

I posted a Peloponnese trip report on another forum which may help with where to go and what to expect in the Peloponnese.

The Peloponnese is a special place in Greece and has just as much to offer as any island and more!

https://www.tripadvisor.com/ShowTopic-g189483-i1277-k14200587-Peloponnese_Trip_Report_Sept_25_Oct_12-Peloponnese.html

I just did exactly your road trip last month and had no problems. I rented from Sixt at the airport and had a good experience. I chose to rent at the airport instead of in town to make picking up and returning easier and to avoid Athens city traffic. I always use Apple Maps since I find the map interface and directions easier to follow than Google. Tolls are easy to pay with Apple Pay or contactless cards. The roads were in good condition and most places were signed well, except once you get outside of big cities or major sights, almost all signs on the roads use the Greek alphabet.