Good to know!

Indeed, they throttle down to 256kbps when you reach the cap but they'll let you unthrottle for $10/GB. Kinda expensive if you ask me, but might be a possibility.

Mint Mobile is similar to Google Fi, and their cap is higher (35GB). But I don't think they'll allow more data to be purchased.

Maybe the real solution here would be to purchase several plans and simpl change them SIM when cap is reached.