Just an update…
Was planning to get some more stuff done post-October but unfortunately things didn’t go that way.
Got the RGB2HDMI boards in the middle of November, and then my partner and I promptly managed to contract Covid (first time for both of us). 
Was not up to soldering or thinking through any sort of debugging processes so I left it alone. Still had a lot of gunk in my lungs after that, so any soldering was out (due to the fumes constantly triggering coughing attacks).
Early December was a bust due to some interstate travel. Got back from that, had a few days to decompress, and then was hoping I might have a few days to get around to things like the RGB2HDMI. Of course, then I somehow managed to get food poisoning. 
Only now just recovering from that.
Not sure I’ll get much of a chance to do anything further till after Christmas, assuming nothing else goes wrong. 