Starting to piece together a 2 week itinerary, looking for some advice!

Found this (haven't read it):
https://www.fodors.com/community/europe/driving-from-thessaloniki-to-meteora-delphi-and-athens-942506/

I will add that the Peloponnese after all of the above would be a stretch in 7 days (from Thessa). Depending on the timing you work out for yourself based on the link, you could perhaps visit Nafplion before Athens departure and after Meteora et al...

Don't miss the Archeology museum in Thessaloniki. Between there and Meteora are good archeological sites related to Alexander the Great and family at Pella and at Vergina.

Yeah it'll be rushed, but hoping to see a couple places in Peloponnese at least (Nafplio and area) and possibly Monemvasia. Is it ok to book accommodations along the way, or should we look in advance? Not sure if it will be like the islands where they recommend to book in advance.

Id say book in advance for sure. Those are very popular places...

Your first day is problematic because you'll have to carry your luggage all around Athens or find a place to store it. Is is possible to just fly directly to Santorini? You could conceivably be there by 1PM or sooner and have nearly a full day there relaxing and enjoying those amazing views.

For the latter part, I suggest you just decide to visit the region north of the Gulf of Corinth and leave Peloponnese for another time. That still leaves you with major highlights like Meteora and Delphi. I also love Parga and if you want to be really adventurous go get lost in either the Epirus mountains or the Pindos mountains or Vikos Gorge.

Islands that could fit into a neat circle would be Lefkada (no ferry - bridge), Corfu, or Skiathos or you could visit the spectacular Chalkidiki (east of Thessaloniki, amazing).

And much, much more.

It is hard to see so many destinations in 2 weeks.

One classic round trip for the Peloponnese is this:
Athens - Korinthos - Mycenae - Thyrins - Nafplio - Epidauros - Sparta/MIstras - Gythio - Monemvasia - Mani villages - Kalamata - Koroni - Methoni - Pylos - Kyparrissia - Bassae - Olympia - Patras - Antirrio - Itea - Delphi - Ossios Loukas - Thive - motorway - Athens.
I suggest you choose the ones you like best and make an itinerary for days you have left.

I'd say you need about 1-2 days for each but 1 day might still be enough. The ones you shouldn't miss are:
1 day: Corinth Canal, Ancient Nemea (ruins + vineyards)
1 day: Nafplio. You can get there after Nemea and have a nice dinner while enjoying the sunset. The next day you can explorethe old town, museum(s), streets shops, Palamidi Fortress
1 day: Mycenae, preferably early in the morning you don't wanna feel all the heat after 12:00
When you return you can go for a swim in Nafplio, there is a lovely rocky beach or get to Tolo or Asine Beach
1 day: Epidaurus to check the theater, maybe combine it with Tiryns (might be able to combine it with Mycenae+Nafplio if you plan it nicely)
1 day: Sparta/MIstras - Gythio
1 day: Monemvasia
1 day: Mani Villages (amazing stone towers everywhere, check Areopoli, Limeni, Vathia)
1-2 days: Kalamata - Koroni - Methoni - Pylos
1 day: Bassae - Olympia
1 day: Itea- Delphi
1 day: Meteora (better get there after Delphi)

Unfortunately, we cannot get to Santorini that day. We land at a strange time and the only flights that we could catch are extremely expensive. I looked into a few storage places around the main square. We have an overnight ferry booked for that night. Not ideal, but it seemed like the best alternative.
Would it be better to jet out to an island on the west coast, like Corfu? We have the rental car now so that might be an issue.

We spent 3 weeks in Greece last summer.

2 days in Athens,
3 days in Nafplio
2 days in Mystras,
1 day in Momenvasia
1 day in Aeropolis to take boats in cave
3 days in Pylos
1 day Messina
1 day Olympia
1 day Delphi -
2 days Meteora
1 night in Athens before heading to Hydra
2 days Hydra
1 night in Athens
Fly home

The only place we would have skipped is Momenvasia. If it truly is the GIBRALTAR of Greece, I will never go to the real Gibraltar. It is a large rock, but a very tiny tourist trap with no good places to swim--the only night I was disappointed by Greece. Spend an extra day near Kalamata or Delphi.