You’ll need to start it as root, and run it as a user that has write permission to the db_dir directory, and has read permissions to the whole mp3 library.
Quick answer is “chmod 664 -R /path/to/mp3/files”
That gives everyone read access to the mp3 files. Then you can
“chown run_as /var/cache/mt-daapd”
Where run_as is your run_as user.
That’s at least a working solution.