" it renames the card as RECOVERY, and reduces its apparent capacity [so the card has to be re-formatted] "
That doesn't sound at all right – but I would reformat on every attempt in any case. Looking at your link re the EEPROM needing re-flashing.. well, in for a penny as they say, you might as well give it a go.
And just to be sure – you are formatting as FAT32, aren't you? Apologies if it sounds like one of those grandmother and eggs scenarios.
Unfortunately I don't have any experience of using a Mac for any of this, but willing to be a sounding board for you. Quite often just telling somebody else about an issue prompts a lightbulb moment in your own mind.
EDIT – having thought about it a little longer, I don't think the EEPROM is the issue, unless the test they describe also shows it to be faulty. I don't think you'd get as far as the rainbow screen if it was.
Edited By Grindstone Cowboy on 11/02/2020 20:36:32