Booking on the fly can be "iffy" as you don't want to waste valuable time trying to find an accommodation that meets your needs and budget.
If you go to Gythio I can highly recommend Studio Gelly's just a short walk from the waterfront. Nice basic studios with balconies overlooking the sea and coast line. Even though they have a website (https://studiosgelly-gr.translate.goog/?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp) you'll have to contact them via their facebook page (https://www.facebook.com/StudiosGelly/)
I've been researching my trip to the Peloponnese next April and yes it is "challenging" but also enjoyable, fun and educational doing the research.
I've been using my Cadogan Guide, Bradt Guide and various websites for research. Also a map of the Peloponnese.
Also using Google Maps to determine have far and how long a drive it will take to get from Point A to Point B.
I've several areas to concentrate on and each would work but I'm also considering budget which always is a big factor on accommodations and car rental.
For me a car is vital for the Peloponnese and am resigned that it will increase the budget but if I'm going to Greece I want to see as much as possible even if the budget gets "bumped" up a bit.
I go to markets for food for most breakfast and lunches and go out to eat at night and maybe an occasional lunch somewhere.
While I'm not cheap I am economical and don't need a over-the-top room when I'll be spending more time outside of it. Still you can get some very good accommodations at much lower prices than I would pay here in the US.
You still have a lot of time to figure out where you are going so take the time for research.
Plus there are some very good posters here who can help with questions and advice and that can be the best help in deciding where you are going.