however when I rebase to that image it drops me into rescue mode after reboot. :(
Did you rebase to the unsigned uBlue image first?
rpm-ostree rebase ostree-unverified-registry:ghcr.io/ublue-os/startingpoint:latest
This will install the proper signing keys and policies and prepare you to rebase to a different signed image. After you run the above command reboot, and then rebase to the actual image you want to rebase to.