mnaag Are any developer options on? What if you set the screen timeout to a long value? Does it crash less if it's fully charged?
Overall this still sounds like a hardware problem. Maybe a developer will read the thread and ask for system logs, but also maybe not. This is just speculation, but if there is only one report, from an EOL device, and the developers have said they believe people should stop using these devices for GrapheneOS, I can imagine they might think it's more important to spend their limited time working on bugs reported by multiple users of in-support devices. Please note that I do not speak for the GrapheneOS project.
If you want to keep using this device no matter what, you might see if the situation improves with a factory reset, or flashing the device back to the stock OS. If it reboots randomly on the stock OS that would be very strong evidence of a hardware problem.