In your situation, the most suitable solution is one of two things:
1) Copy 25GB of music to Documents6 through one of the computer applications (iTools, iMazing, iExplorer, iTunes), connecting the iPad with a wire to the computer. If you do not want to transfer all the music at once to the iPad,
2) Buy one of the lightning flash drives, the application of which can send a file to other applications in iOS through the Action menu (not all applications can do this
But in this case, you still have to copy 25GB from the previous flash drive to the new one using a computer.
To date, I know for sure that the following models are suitable:
Leef iAccess iOS microSD Reader
or from dan_house info
I do not advise buying a wi-fi disk or a Wi-Fi card reader. In addition to the fact that it require self-contained power, less reliable, more complex and slower in comparison with a lighting flash drive or lighting card reader.
And finally: I hope all your music is not stored on one single flash drive. You definitely need a backup on another media.