Balboa GS523DZ
Some Balboa SPA controllers expose a RS-485 connection (to add on a Balboa Wifi module) which makes it possible to communicate with it. Then the following types of projects are a possibility:
https://github.com/ccutrer/balboa_worldwide_app/wiki
https://github.com/cribskip/esp8266_spa
The Balboa DS523DZ does not have any RS-485 from what I have understood, so we are left with two other options:
There is a connection for "Dolphin Remote" which seems to have some kind of SPI bus which could possibly be used:
https://www.olivierhill.ca/archives/72-The-Internet-of-Spas.html
The third option is to intercept the signal that goes between the mainboard and the display unit:
https://docs.google.com/document/d/1xuc5N6-Q7J7bQxBARYjUywhqkIFjOnh8vAHqEya5QAI/edit
https://www.raspberrypi.org/forums/viewtopic.php?t=175399