Mainland loop and island hop. Please advise.

Hoping to one day retire and do the same, so I am wondering if you planned some longer stays to be included (like the week in Naxos), would that reduce the feeling of "too long?" Your plans look pretty good--of course there are things you are missing, but it's a nice overall plan.
Making it two trips adds so much airfare costs to the plan that I would have a hard time with that (though in the big scheme of things, I guess it all balances out)--but if you did, I would do mainland in May/June and islands September into early October for sea warmth.
Another random thought--keep one trip as the Peloponnese/Athens and islands, and make a second trip northern Greece plus some islands (maybe just Crete).

Regarding the mainland, I know it's necessary to define stopover locations, but don't make the mistake of confusing the must-sees in travel guides with these stopover locations.

Places like Olympia, Monemvasia, and Delphi aren't necessarily places to stay overnight, but rather places to visit, where the visit can be as short as 3 hours. And if you do as I usually do and carefully study the route to take between two stops, you might discover tons of places to spend time, enough to turn a two-hour drive into several days.
Greece isn't just a series of must-sees; there are hundreds of things to discover in between (I'm taking advantage of the fact that you have a lot of time to try to convince you and, if necessary, transform your tourist side into a traveler)

I see you've only considered the Cycladic islands. Since you enjoy nature and hiking, you could add Kythira to your Peloponnese itinerary.
It's easy to reach in a 2-hour ferry from the ports of Gythio or Neapoli (a 45-minute drive from Monemvasia).
See some photos:

https://photos.app.goo.gl/fq4yiQ4Jz4FpFqWL8

Regarding the "islands" section, if you're traveling in May, you may not have daily ferries between two islands in the Western Cyclades (Milos-Sifnos-Folegandros), or you may have ferries that run very late in the day. It's worth considering.
But otherwise, your itinerary is pretty good. I could add a few more islands and ask why you're only going to the Cyclades, but I'll refrain.

It's up to you to see how many days you spend on each island, knowing that you need at least four full days on an island, even the smallest ones. Santorini can also be worth four days if you're not just following the crowds of tourists who are short on time.

If it's at all possible try to get to the island of Kythira as it's a wonderful island that doesn't get the hype of the Big Name Islands.

It's do able from the Peloponnese and gives you a chance to experience a more traditional island without hordes of tourists (off season anyway) and probably even during high season.

Lots to see including lovely villages, archeological/historic sites, secluded beaches, easy walking and more challenging hiking routes but offers something for everyone.

Check out my trip report with photos:

https://www.tripadvisor.com/ShowTopic-g319777-i11468-k14565904-Kythira_Trip_Report_Sept_28_Oct_3-Kythira_Attica.html

Thank you all for your responses! I do appreciate what you experienced travelers have to say and I consider it all when finalizing a trip.

@valadelphia - I am looking forward to your retirement so that you can travel when you want for however long. :-) I do agree with you. I think a longer, more restful stay in the middle of travel, especially a long road trip, could be helpful. We usually take Rick's advice and have a 'vacation within a vacation', but the trip was already pretty lengthy compared to our travel in times past. Even when I built in 'relaxing days' we hiked for hours. That just means I'll have to keep traveling and tweaking. :-) By the way, I do like your thought of northern Greece plus Crete as a separate trip. That looks interesting!

@JoLui - Thanks for your comments and questions. I completely agree with you that, to me, it looks like Olympia, Monemvasia, and Delphi are more suited as places to visit rather than stay the night. Having just come off a 5-week road trip in Central Europe, my husband and I definitely studied and focused on what there was to see between the point to point places. On travel days, our habit was to have a leisurely breakfast, check out by 10a, and meander to our next stay, arriving late afternoon/early evening. That's why we like our next stay to be, ideally, 2 hours away. Whether it's the countryside, a gorge hike, a castle, an abbey, or a scenic mountain pass, we enjoy the in-between as much as the locations of the stays themselves. Perhaps I need to find better stopover locations in this case. For example, I would love to see Delphi on the way to Meteora, but that's a very long drive on our arrival day. We won't drive more than 3 hours on arrival day. Ideally, two.

You asked why only the Cycladic Islands? I kept reading that it's best to travel in the same island group for ferry planning purposes. I will take a look at Kythira! Thanks, tommyk5, for mentioning that island as well.

All in all, I do want the Pelonponnese road trip to be our priority and I can certainly make tweaks to that loop at this point as well as not see every island on my current hop. My husband has told me before that even though every place has something different to offer they can start to all look the same to him. I think that comment came after I took him to most of the white towns in southern Spain. ;-)

Please keep your comments coming and I'll follow up! Thank you!