Northern Thai Variants

I have been looking at localisation for northern Thai (language code
nod) and it seems to me that, simplifying, for any speech variety there
are three basic varieties of writing system:

1) Writing in the Tai Tham script (subtag Lana)
2) Writing in the Thai script (subtag Thai) with consonants having
the same phonetic values as the corresponding Tai Tham consonants.
3) Writing in the Thai script with stop consonants having
the same phonetic values as in Standard Thai.

My idea for approaching this is to use the labels:

nod-Lana
nod-Thai-etymo
nod-Thai-phonetic

However, there are several issues with this approach.

The first is whether nod-etymo and and nod-phonetic would then be
allowed as identifiers of Northern Thai Thai-script writing systems.
nod-Lana-etymo would not make sense, and I'm not sure how to word the
subtag record to exclude, or at least discourage, it.

The second is the possibility that Scheme 2, but not Scheme 3, should
be treated as a transform of Scheme 1, and therefore scheme 2 should
be nod-Lana-t-Thai, which would remove the need for variants at this
level.  My most cogent objection is that the transformation depends on
phonetic and etymological information - ambiguities in some
orthographies for Scheme 1 are removed in Scheme 2, and word-final
<SIGN OA BELOW, RA> is converted according to etymology, (generally)
with <O ANG, RO RUEA> in words of Pali/Sanskrit origin and <O ANG, NO
NU> in native words.

Users of Northern Thai learn Standard Thai in school, and will
generally be more fluent in writing Standard Thai than in writing
Northen Thai.

Can I please have advice.

Richard.