Please don't say iTunes. I've owned countless iPods (including the original) and iPhones (including the original) and iTunes has never been a reliable means for getting content on to an iOS device.
I basically have a bunch of mp3s (~4,000) that I listen to on my desktop & my iOS device. I regularly run into the "dotted circle" issue, resulting in only some of my music being transferred.
I've searched & tried the various tricks on Stack Exchange, as well as elsewhere on line.
I've reset my device but this is not practical to do on a regular basis and it hasn't worked. It results in a different, random subset of my songs being transferred.
I've tried removing all my music & resyncing. This doesn't work. It results in a different, random subset of my songs being transferred.
At this point, I want to give up on iTunes.
I just want something to get my music on my iOS device & enable me to listen to it.
I'm not interested in cloud services, despite Apple's push. I already own most of the music I want to listen to & I'm not into new music. I already paid for the music I have. It seems silly to now "rent" it.
I'm fine with not fancy. I have no need to for fanciness. I don't care about machine learning, auto playlists, content discovery.
Again, I just want to transfer & listen to my music.
I'm totally fine if this is requires new apps. As long as it transfer music & there is a "play" button, I'm happy.
Similar questions from my research (that don't actually help in this case):