I can (almost) confirm my heater is just as you're suggesting. There's a circuit I need to close to turn the heater 'on'.
I've got the SONOFF TH16 that I'm going to use to close that circuit, as it supports a submersible temperature sensor DS18B20 right out of the box.
I'll integrate the temp sensor and heater control today, and update the schematic.
is mqtt an option?
if so, I used this setup for my aquarium: https://www.amazon.com/gp/product/B06XW6STL7/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B06XSHMG8P/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1