Thanks for the response! So brainstorming it, maybe the best way to handle it is have an dialogue option at the start of the game that sets the switch for which OST to use? I think you are referring to something like that. I don't know code (why I use other people's plugins), so wouldn't know how to set this as a menu option. Especially since I use VisuStella for a lot of stuff, including the options menu core.
I will actually give your plugin a try here then. And if you have any further tips on how best to handle this, it would really be appreciated!
EDIT: Hmm, so I got it working sort of, as far as having the music playing on the overworld based on a choice menu I have at the start of the game. And I set it to change battle BGM if it is set to synth, otherwise not to change it from what's set in RPG Maker MZ settings. But, when it transitions from overworld to battle, the overworld music keeps playing.
EDIT2: Okay, I got it to play the proper track in battle by adding it as a plugin command to the troop on turn 0. If there is a cleaner way to do this, let me know. New issue I have encountered is, the battle music and overworld music are playing at the same time in battle, I don't know how to make the overworld music stop when the new track is set to play through your plugin. I have a bit more time to fiddle tonight, if I figure it out I will post back. Since all of this might help someone else who wants to do this as well.
EDIT3: Okay, for that issue, I set it to crossfade when playing the new music, and a crossfade timer of 0. Making progress! The victory music isn't playing though. It sounds like it fades out temporarily where the game wants to play it, and then it immediately resumes playing the battle music. I'd also need to have it be able to choose between 8-bit or synth victory music, so not just what it's set to in MZ Database - System 1 tab. Should have more time to look into it myself tomorrow. Thinking about it, I guess I would also need it to be able to switch to game over music and stop battle music on that happening, and properly play either 8-bit or synth music.
EDIT4: Sorry for the wall of text! Hopefully this isn't overwhelming. I decided to try a bit more. But, it seems it is basically not really triggering music effects. For example, I have a recovery statue on the map, and when you use it, it's supposed to play a jingle. With this plugin active and the overworld music playing through the plugin, the jingle doesn't play. If this is too much, I understand this might be beyond the scope of your plugin, let me know. As cool as it would be to have this working, I had initially planned on only having 8-bit music, so it isn't like the project is hopeless if that's all I can do.