thanks for the detailed followup! so, I was using the BNROM version on original toploader NES. I think it's maybe at least worth trying the GTROM version next so I'll give that a go and let you know :)
I saw that some users were having issues with the latest firmware, so I rolled back to v1.16. But either way all I get is a gray screen on boot. Other ROMs boot without issue - is there something else I should try?