This app was mentioned in 2 comments, with an average of 1.00 upvote
I'm so glad you found the same as me, no decent app on android for listening to your own music files.
You mentioned you have a NAS, you could try ds audio if you have a Synology. https://play.google.com/store/apps/details?id=com.synology.DSaudio
But totally agree there's an app needed desperately for this.
Synology's DS Audio app! https://play.google.com/store/apps/details?id=com.synology.DSaudio