gdmplc66. I know that answers to requests like yours often include "you can't do that," "you won't have enough time," etc., which initially generate disappointment and frustration. These answers shouldn't be interpreted as obstacles to travel, but rather as ways to help you plan your trip.
Understanding the constraints of ferry travel between the islands allows to narrow it down to what's feasible within a given timeframe. Given that there are roughly 20 islands worth visiting in the Cyclades, choosing 3 or 4 islands for a 3-week trip shouldn't be difficult once you're aware of these constraints.
But since we're not in your shoes, it's difficult to say whether you'll like this island or not. So, to keep it short, I'll just mention one route, deliberately leaving out Tinos, Paros, Naxos, and Koufonissia.
To summarize, the western Cyclades accessible from the port of Piraeus are, in the order of the ferry routes: Kythnos - Serifos - Sifnos - Kimolos - Milos - Folegandros and Sikinos. Not all ferries necessarily call at all these islands, and some ferries continue on to Ios/Santorini.
Therefore, it's a trip you can study by looking at what each island has to offer.
Given your preferences, I would focus on Serifos - Sifnos - Folegandros. This makes for a fairly varied mix of landscapes and atmospheres. But be aware that there won't necessarily be a ferry every day to go from one island to another. This needs to be checked on websites like ferryhopper or openseas.gr.
You should also consider the possibility of ferry cancellations due to bad weather, technical issues, or strikes. In that case, you could be stuck on an island for an indefinite period. Booking accommodations with flexible cancellation/modification options is a good idea.
Serifos isn't on your initial list; So see if it appeals to you. The island is mountainous, so a car or motorcycle is highly recommended given the rather limited bus network and the limited access to the southern beaches and the northern part of the island beyond Chora, the capital.
https://www.discover-serifos.com/en/
Sifnos is a hiker's paradise and the quintessential Cycladic island (whitewashed houses, blue domes, beaches, and everything you see on postcards). Its bus network is excellent. The main bus hub is roughly in the center of the island, where the capital, Apollonia, is located. The south of the island has charming seaside villages, and the ancient capital, Kastro, is a must-see. Sifnos is well-known among food lovers. At the end of September, a gastronomy festival is held there.
To find accommodation and other information about Sifnos, check out:
https://www.roomsinsifnos.com/
The website is managed locally.
Folegandros is also quite mountainous and very peaceful. Its main areas to stay are basically Karovastasis (the port) and Chora (the main town) in the middle of the island.
It's more of an island for walks and hikes than for spending time on the beaches. This might be why it's on your list. Transportation in Folegandros is basically limited to a bus from the port to the capital, hence the need to enjoy walking.
https://www.folegandros.gr/en/
Once in Folegandros, you'll be ready to head off to the crowds of Santorini; it will be a whole different world.