Small cruise ship tour of the Greek Isles?

Check out Variety cruises.com. We "cruised" with them before Covid. It's either a 20 or 24 cabin Motor Yacht and we stopped at islands the big ships couldn't dock at along with the major islands, and it's way under your price point.
Also, we did the same kind of cruise on the Kelopatra (?sp) from Split to Dubrovnic and back. Both are EU operations, Variety, Greek, and the other Croatian so you'll be paying inEuros rather than dollars.

Peter Sommer Travels may have something that meets your criteria. Some friends of mine just returned from a land-based tour of Greece with Peter Sommer Travels and speak v. highly of the experience. There are two cruising Greek trips and one looks to me to be island oriented.

The most obvious choice is Windstar. Our ship missed Mykonos because of a storm, we stopped at Monemvasia instead, VERY satisfactory replacement. Cruise ships, even tiny WIndstar's tend to go to "the biggies", not the "slow-ferry-only live-local, peace love tie-die" places Rick might recommend. Oh wait, summer vacations are .... tourists! There's also a Windstar route that does the Corinth Canal, which is more impressive for engineering for scenery, I think. Typically 300 passenger ships. You want a degree of size and organization to cater to your needs. They might still have one ship without an elevator. (?)

Happy upcoming Anniversary!!!

Check out Silversea. Very nice high-end small ship brand with whom we have sailed three times. Ask for Lisa if you book with them. Age-wise, you would fit right in. Pulls a generally tame, well-traveled group of people. If I were cruising again, I would definitely go with Silversea (if not doing a more expedition-type cruise with National Geographic/Lindblad).

Then for a true sailing ship (that will likely operate with its engine most of the time and just put up the sails for certain periods and photo ops), checkout the SeaCloud (the original historic one that was a wedding gift to Meriweather Post from EFHutton)....seems I recall approx 68 guest passengers tops.

We sailed Greece/Turkey with Silversea several years ago, and totally enjoyed the experience...on a 240 passenger ship (which I think now uses a bit larger ship).

(ooops...I just re-read your post and see your wife's mobility issues, so I would nix my recommendation for the historic SeaCloud....old-style stairs on that historic ship would likely be very challenging...but I will leave what I originally typed below, in case others are looking for similar advice.

We took the SeaCloud on a short Venice to Venice sailing a few years ago. I posted a review on the RS site after that trip. My basic recommendation ( which we did not do, but would if we had a "do-over") would be that IF you take the SeaCloud, do it thru National Geographic/Lindblad. While likely more a bit more expensive, my guess is that it would be a better experience (NationalGeo naturalists/guides would be on board and fellow passengers would be easier to interface with..most would be English speaking...during meals). Our journey had predominately German-speaking passengers (SeaCloud is now German owned), and while everyone was delightful and they tried to include us socialily, at dinners the language naturally reverted back to German after some initial English-speaking phrases, etc. One of our shore excursions, while we signed up for English, was less than desirable due to completing with German shore excursion commentary blaring over the bus microphone while our English-speaking guide huddled with the few (as in 4-5) of us English speakers on that excursion.

Well, here you go, just to begin to get a feel of what's available, (one of several Silversea voyages listed that include the Greek Isles):

https://www.silversea.com/destinations/mediterranean-cruise/athens-piraeus-to-athens-piraeus-sl230625007.html

While the cost is over $5000 per person, it is listed as "all inclusive," including airfare, meals/drinks, butler, and one shore excursion per person per port. Room service is also included with Silversea (and I love getting breakfast that way on early shore excursion days, or dinner that way after a long day touring). Also, unlike many big ships, there are no assigned seats for meals, and you can choose to dine under the canopies outside to really enjoy the views.

In the past, sometimes when Silversea offered various promos, such as air included, but if the passenger already had air taken care of (like using airline points, etc.), then the price could come down. Also, in the past, sometimes when one flew in a day early, there were either included or specially priced hotel arrangements available. Just ask Lisa what is available and find out what best pricing they can offer. Also, if they know if is your anniversary, expect a bit of extra pampering.

And, lastly, what we also love about Silversea, no need to lug the tux.