> My main mistake seems to be meaning “[Hepburn] romaji” by writing “romaji”. I was obviously aware of other systems because that is what the sentence says but I thought it’s acceptable to refer to Hepburn as just “romaji” as a sort of the default one.
"Romaji" does not (in English) mean "romanisation", as most people who've studied Japanese to at least beginner level know.
“Romaji” in Wikipedia literally redirects to “Romanization of Japanese” which says “This method of writing is sometimes referred to in Japanese as rōmaji”. It sounds like your effort might be better spent fixing those problems in Wikipedia before continuing with the rest of the discussion here.
> "Romaji" does not (in English) mean "romanisation"
> This method of writing is sometimes referred to in Japanese as rōmaji
See how there's not actually a contradiction there?
Can you just explain what you’re trying to say. I genuinely don’t understand your point. What is my mistake?