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As you are probably aware, Google is trying to kill the SD card and you can move fewer apps to the SD card and even doing so, its only part of the app, with that said, indeed both the M4 and G 3rd generation have quite small limited internal storage.
The google music issue, will have to be addressed by Google music developers, or if I misread please elaborate.
The Camera folder should be
SD card/Internal storage > DCIM > 100andro for regular pictures, if you use another camera setting then it'll be on the Xperia folder.
If you check their Nexus devices they lack SD card support bc they want you to buy phones with larger internal storage, back in the days, apps to SD card was a great feature but then Google killed it after ICS and it was "resurrected" not too long ago but as you can see it's not ideal nor Google has explained the reasoning behind it.