More discs to find

You could have themed music discs for more places(end cities, mineshafts, nether fortresses, etc.). You could also have them as rare drops from certain mobs(elder guardian, wither, phantom, husk, etc.) and as a rare fishing drop. You could even make discs that you can craft with semi rare items. If there were discs in special structures and biome specific mobs there would be a better reason for late game exploration besides filling out achievements.