I will give this a crack!
As far as I am aware there is nothing decent that allows such filtering by ship size, classic/expedition, solo discounts etc. Solo discounts get offered and pulled quickly, usually at the whim of the cruise line, so it's hard for third party sites to include this filter reliably.
This is how I would work through it.
Preliminary search on Vacationstogo using their old school Search Box. Ignore their "bargain" prices and marketing. They often don't include taxes and fees and are for US residents in US dollars. Just use the search box to identify suitable cruises.
In this case:
July 2026 - Oct 2026
Greek Islands
You can also be port specific which is really handy.
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Then one by one filter down to the relevant cruise lines . Yes, this takes product knowledge of ship sizes and what they do. I've included lines that carry 700 and under below, that have non-expedition ships.
Ponant
Silversea
Seabourn
Emerald
Windstar
Ritz-Carlton (if you're loaded)
Regent
Oceania
Azamara
From their go to specific cruise lines websites and see how it fleshes out for a solo on the sailings you're keen on.
Most solo deals are for travel within the next 6 months when the cruise lines has enough inventory to offer these deals. And most cruise lines have a "solo" page these days that identifies all current "solo" friendly cruises.
Some examples. Ponant
here. Silversea
here. I bookmark for easy reference.
There will also be a few cruise lines that are missed in above search, such as this
one with Travelmarvel.
Hope this helps.