Hi Charles,
i know its not your mistake. Since the new Board Version ESP8266 3.0.0 the connect to MySQL crashes every time. I am not the professional to find out what is different to version 2.7.3. That was my last attempt with which version the "connect" function worked. Could you inspect this issue and modify the library? You could use one of your example and you see what happened.
thx
Falke