Thanks for this - you saved me hours of searching.
Bumping in case anyone else has the same problem but also because some messages have been deleted and the solution was not clear to me initially.
I was also installing from a Pixel Tablet to an 8a. I had the same situation where the install just hung at "restarting device.." but there were no obvious errors and the phone was idle at fastboot.
It appears as if the two devices disconnect and so the process stalls. The solution, as stated by Ooops, is to reconnect by pressing the "unlock boot loader" button a few steps back and selecting the device in the following pop up. This kicks the install back into action and you can see the OS flashing progress bar. This needs to be done each time the phone reboots.