Yup, I would say you covered all possible options with this one.
Considering it’s the original Prime 4, as you say in perfect condition, with very old firmware out of the box - I would say this could be a hardware problem connected to unit not being used for a very long time. Reminded me of this topic TBH: Bricked Prime Go Support Needed - #33 by Soulframe
@Soulframe did you sort it out in the end and how?