MQTT does have its nuances, and diving into the specifications is sometimes required to grasp all the options fully. That said, for most cases, simply focusing on the API for the library you're using should suffice. It sounds like the real issue you're facing is more with the library than the protocol itself.

If you're open to using Lua, you might find this client library more straightforward: https://realtimelogic.com/ba/doc/en/lua/MQTT.html. It’s designed to be simple to use and works with the BAS runtime, which supports many platforms, including microcontrollers like the ESP32: https://realtimelogic.com/downloads/bas/ESP32/