I'm surprised this is legal. Number portability isn't something phone companies offer out of the kindness of their heart; it's required by law. Does the law really allow them to hold the number hostage as part of a payment dispute?
EDIT: Nope, this is illegal:
> Once you request service from a new company, your old company cannot refuse to port your number, even if you owe money for an outstanding balance or termination fee
https://www.fcc.gov/consumers/guides/porting-keeping-your-ph...
I looks like the OP should file a complaint (and if necessary sue?) over this point.
Indeed, this is so clear cut it makes me doubt the OP's story. Does Google Fi say they do this anywhere?