For anybody who's interested:
My (reverted back to stock) Mac mini was ready for collection from the Genius Bar today. Apple admitted that there was a "Hardware fault" with my Mac mini and replaced the logic board FOC since the mini is in warranty.
When asked, they wouldn't give me much more info than that, but if I had to guess I would shoot my shot at the T2 chip being faulty as this explains a few issues I've had with the machine.
I've decided to keep the 8GB RAM for the time being -- if I get a few weeks in and the Mac is running fine but needs the extra RAM, I will consider upgrading again.