Too many to quote here, so I'm picking the latest and trying to address everything I have read. Strap yourselves in, boys!
I have used VLC and PowerAmp. VLC on PC PowerAmp on Android. The 4.99 I paid for PowerAmp was among the best app purchases I have ever done. Toss up between that and Nova Launcher Prime/Pro. VLC...never really cared for their UI. But I will wholeheartedly admit, who cares if it looks ugly as long as it works. And it works well.
As for music organization and playing...I have a dedicated folder for music (and subfolders for artists, then albums) on one of my HDDs, and I actually use Windows Media Player (the newer one; looks like something they stole from PowerAmp), and link my HDD folder to that. Since everything is a standard format, I have had no problems playing.
As for Android, I just drop the artist folders I want into a Music folder on my phone and link PowerAmp to that folder. Boom, I am playing music with one of the best apps for an equalizer I have ever used. (actually, to keep things simple, I have a Pixel folder of music on my desktop. Essentially a copy of what is in my main folder. This way I can easily trash it and start a new playlist over)
PowerAmp works quite well with Android Auto that is preinstalled on my factory head unit (2016 Ford Fiesta). The interface is nice, large buttons if you choose to use the on screen display for input.
I'm not sure what to suggest about the library on iTunes. If they are all mp3s, you may not need to do anything.