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The oldest unmodernised onsen I've been in was built around the 1750s, so hardly a perfect analogue, but it had a pretty similar styling and layout to what you've planned. The only sìgnificant difference was that the spring pool fed into a split bamboo chute, which diverted the water into a large barrel bath- to regulate the ~65°c water that comes out from the ground and get it closer to a 40°c-ish hot bath temperature. Otherwise your layout feels really good, the amount and size of buildings is spot on 👌
Lovely, I may borrow some of these ideas if that's okay
it lacks water all that
That's just begging for a miniature of a guy painted bright red from the neck down, wearing just a towel on his head and a loincloth type affair to preserve his modesty 😰