MP3Gain, the latest stable version was released in 2005 and I’ve been using it since then. It adjusts the volume level of MP3s using ReplayGain so that you don’t have to change the volume between tracks. There may be newer software out there that does the same thing, but MP3Gain is free/open source and it works fine for my needs. I still maintain a library of MP3s, partly because my car is too old to do Carplay/Android Auto, but it can play MP3s from a USB drive.

For android I was using Jerboa but switched to Liftoff and am liking it so far.