I can recommend Node-Red to read your API (ask their support if they please write a nodered library, else play with http- and function-block) on the one side and send HTTP GET commands to OS: https://openthings.freshdesk.com/support/solutions/articles/5000716363-os-api-documents
If you were to use a RaspberryPi for that, I’d advise using IOTstack: https://github.com/SensorsIot/IOTstack (NOTE: do install RaspAP FIRST!!! https://raspap.com/ )
Also think and read about the backup function BEFORE you go into production.
PS: You could ofc try to modify the firmware, but for that you’d have to read into it first. For that I can recommend opening the firmware in PlatformIO and CTRL-clicking terms of interest to reveal their definitions or flat out searching for terms like MQTT, HTTP or STATE. Node-Red is the beginner friendly way, but it also has many pitfalls! Nevertheless there are many people who solved similar nodered problems on the internet, compared to OS-compile-problems.