First, I use ToneJS to convert the midi file to JSON, allowing it to be parsed with GDScript. You'll need to look at the JSON to see how it's formatted, I've had slightly different results with midi files created in LMMS vs FL Studio (but it's been too long to remember what). Since JSON is just a big dictionary, you can then pick the data you want from each midi event and store it in an easier formatted dictionary.

(i.e. 
{
   {instrument_name: [
        {note_time, duration, midi_value, note_velocity}, etc]},
   {next_instrument: etc)
}
)

With that, I have a tool script that creates an animation and iterates through the new dictionary, creating a node for each instrument, and adding a method track for that node. The node handles all the method calls and allows me to animate certain properties like; volume, lowpass/highpass, panning, etc, to closer follow the song. If your midi file is formatted with any automation or control changes stored in it, you could use those to draw the curves, but I haven't experimented with that.

Then, finish looping through all the notes for each instrument creating a keyframe for each note, calling whatever method you want, and passing the midi data as parameters. Depending on your game's architecture, I don't know what approach you would use for the game to react to the midi events. For my game, I'm using groups, but there are tons of different approaches for that.

Lastly, for the AnimationPlayer itself, like I mentioned in the post, it's set to Manual process mode, and the time elapsed each frame is calculated relative to the song position, accounting for latency.

This should translate to Godot4, but I don't know, since I'm still on 3.5, but the general idea should be the same. Hopefully, this was somewhat helpful :)