With an apple device you have to use iTunes to transfer music, however with some audio players, you can just use iTunes to copy the files you want in the player’s folder. So you don’t have to use iTunes library at all, but still need to use iTunes to sync files with the audio app. As far as I know there’s no other solution for flac files for example.
First, thanks all for your replies. Glider does seem nice, and ticks a lot of boxes for me. (lastfm, and can control my local MPD machine).
So, you can't get around the iTunes Library sync. You need either iTunes or maybe a couple other of specific apps (I think Waltr for example?) to populate the database on the phone with music (and in the process I think will convert flac to e.g. alac). Apart from the file transfer itself, they also perform some db work on the phone so that the music is recognized.
iTunes file transfer is a different thing. Apps that implement local storage will allow having files transfered there without conversion. However, because there is no db that needs updates etc, I can do that even from my Linux pc using libimobiledevice. Since I don't have access to a Windows or Mac (so I can't use iTune Library), the only sane solution for me is to pick apps like Glider that do that. I already tried with my wife's iPad Mini 4, and I can also get access to DCIM folders for copying pics/videos etc. (However, I haven't tried it after the ios 12 update).
Even without plugging the ios device on a PC, using the Documents app I can access my pc via sftp, pick some flacs and add them to Glider. (Tried that also briefly yesterday).