I've been on Google Fi for almost a decade. $20/month + $10/GB until you hit 6, then the rest is no cost. 99% of my usage is on wifi which comes with no charge and my bill rarely goes over $30/month, only when travelling.
One of the most underrated features of being on Google Fi with a Pixel, you don't get any of those apps like Amazon and NFL you are unable to install.
I have yet to try GrapheneOS primary support is for Pixel phones and looks like Fi can work.
https://discuss.grapheneos.org/d/7950-does-grapheneos-work-w...