ISO 15919 has two options about anusvāra. (1) In the simplified nasalization option, an anusvāra is always transliterated as ṁ. (2) In the strict nasalization option, anusvāra before a class consonant is transliterated as the class nasal—ṅ before k, kh, g, gh, ṅ; ñ before c, ch, j, jh, ñ; ṇ before ṭ, ṭh, ḍ, ḍh, ṇ; n before t, th, d, dh, n; m before p, ph, b, bh, m.
It would be nice to support the strict option I think.
Quote from https://en.wikipedia.org/wiki/ISO_15919#Comparison_with_UNRSGN_and_IAST:
It would be nice to support the strict option I think.