Why did you choose to not use the Arduino IDE setup() and loop() code?
Last time I ran DFPlayer Mini (project interrupted by COVID and moving house, code ... better to use what is forum-archived here) I used Arduino IDE setup() and loop() with Serial to Serial Monitor interface and software serial to the player. That let me read the messages the player sent back and not need esoteric means to access the player.
There is a library according to links I saw in a web search.
There are docs for the player including how to run it stand-alone with buttons and what buttons can do, Arduino IO pins can do for simple things like you want.
Just one answer, not a complete one but it's more than you have.
I dunno why you chose to go the route you did. Maybe that's what you're used to or maybe it's what you found on the net. The difference means a lot as to what you do now.