useless hot take: instead of reusing n (and having to add an apostrophe to resolve ambiguity), rōmaji transcriptions should use a tilde over the vowel ã for any 2-mora syllables ending ん whenever they are followed by a vowel or by s/sh/h/f/w. For example,
õyomi vs on'yomi
kũyomi vs kun'yomi
gẽin vs gen'in
The tilde indicates both lengthening and nasalization. This notation is much cleaner and matches the actual pronunciation (if you speak portuguese hehe).