How to desolder a BGA 153 eMMC chip
The easiest and less destructive way would be to turn your board upside-down, and heat the chip area from the PCB side. Use some powerful heat gun instead of a tiny hot-air pencil. As soon as the solder temperature gets to the soften/melting point (which you don't know and shouldn't guess), the chip will fall down by itself, maybe with a little help.

You can't afford re-balling, plus you will need to develop a PCB interposer for the BGA with breakout pins, etc. Best way would be to use a very thin magnet wire, AWG36 to AWG40, and break the connection to some DIP-40 socket, or something. Solder 60/40 would be just fine, if you have a 10X stereo stand and fine-tip soldering iron.

ADDITION: Here is your other option without soldering the chip back:



You just need to clean your eMMC thoroughly.