@Dusty I am in the process of flashing GrapheneOS from my Pixel 8 to a Pixel 6a using Vanadium, to see how Vanadium behaves. There's one thing that caught my eye that I'm not seeing mentioned so far: When the "Restarting device" message is shown, to the right of that message there is a blue button called "Reconnect device". Each time the phone reboots during the flashing, I have to press that button to confirm access. Are you seeing this button on the Tablet? Screenshot.
If you don't, there might be some kind of scaling issue at play here, which is somehow hiding the button outside of view. In that case, you could experiment with various settings in Vanadium settings > Accessibility and/or in Settings app > Accessibility > Display size and text.