drtweakllc For me the graphics test runs for around 15 seconds (it isn't doing an update; it's just showing you status screens from an update process to exercise the screen a bit). The locale test will display a few text items in a variety of languages and fonts -- for me, 79 different things if I keep hitting volume-down long enough. Neither one really exercises the machine a lot. "View recovery logs" will do some work if you keep hitting the volume-down key, but it's not really a hardware exerciser either. If you could get something other than GrapheneOS to crash, that would suggest the problem isn't a broken GrapheneOS install, but hardware instead. But it's a one-sided test: if the limited things you can run from Recovery don't crash that doesn't mean there isn't a hardware problem.
Have you tried to boot GrapheneOS while the device has external power? That shouldn't matter, but it's something that could be tried. I guess you could also try chilling the device for a while before booting GrapheneOS. Again, that shouldn't matter, but maybe there is a heating problem?
BTW I think the "tunefs" message you see isn't diagnostic. I have seen that message too. And I found an entry on a LineageOS page that suggests the SPI message also isn't fatal.
When the next GrapheneOS release comes out for your hardware you could try installing that via Recovery (downgrade protection doesn't mind at all if you move forward).
But I think I, personally, am out of ideas. Maybe try the Matrix chat room again?