Hmm, my motherboard (Asus) just has a reset button that flashes the contents of the secondary ROM to the primary ROM. If there's any problem getting the primary to POST, you just push the button and it reflashes the primary from the secondary, so you're back in the original state. It's not a one-shot, because the contents of the secondary don't change. You can use it as many times as you need to.

Once you get a good POST from the primary, it updates the secondary. I've never had an issue with this system.

Phone mainboards are very different, which is a big part of why maintaining third-party phone OSes is such difficult work, and why GrapheneOS constrains itself to specific devices.