just a little quick research
serbia has really old and famous spas with hot springs and villages near caves and rivers
www.serbia.travel/destinations/spas-and-health-resorts.701.html
this one f.e.
https://banjakoviljaca.rs/ger/banja_ger.php
and in 2017 there was a sale of 10 bathhouses
https://www.ekapija.com/de/news/1619660/zehn-heilbaeder-in-serbien-zum-verkauf-ab-januar-2017
i haven't found any photos of the old baths yet where you can see the interiors