I previously posted in the thread below, and was able to recover my phone using the Rescue Mode Graphene OS recovery menu. Last night, I restarted my phone in response to the latest 10/23 update, and was greeted with the same No valid OS prompt. This time, I just let my phone restart several times and it rolled back (A/B partitions doing their thing, I assume).
Obviously, I'm uneasy to install the update again now. For the moment, I've disabled updates at restart and am trying not to reboot. I may investigate more over the weekend but right now I'm just trying to keep the phone operational. If there is anything that might be useful to the GrapheneOS team, I can try to get logfiles etc from my phone. I'm not super tech savvy, but I have a working adb install if someone can let me know what commands, directories, etc are needed.
I've read that at it's root this is an issue with the Google bootloader. However, it seems this issue is somehow getting exercised by recent updates.
https://discuss.grapheneos.org/d/8064-pixel-7pro-cant-find-valid-os-after-update/16