The JSON string is after the trigger "??MYSQL??", I supposed to recognize this trigger and cut out the string behind it.
The "trigger" "??MYSQL??" is useless. It adds NO value to the server output. The data of interest starts with the '[' and ends with the ']'. Just store the data, after the '[' arrives, and stop storing the data, and parse the data, when the ']' arrives.
The ArduinoJSON library can handle the parsing of the JSON data.