Pitman Phonetics

The other day I came across an alphabet using different shapes of leaves instead of letters. This was a basic substitution of the 26 letters, with no attempt at phonetics or other innovations. It occurred to me more could have been done with the idea. Using the simplest leaf shapes for the vowels and more common letters was an obvious one. The idea that the symbols could be used both stem down and stem up reminded me of the shavian alphabet which used tall letters for voiceless consonants and deep letters for voiced.
Thinking about the shavian alphabet took me to Quikscript, and on the theme of quick writing, to Pitman shorthand.
One thing I noticed in the article about Pitman was that it used six long vowels, six short and only four diphthongs. These were remembered by: “That pen is not much good”  /ðæt pɛn ɪz nɒt mʌt͡ʃ ɡʊd/ for the short; “Pa, may we all go too?” /pɑː | meɪ wiː ɔːl ɡoʊ tuː/ for the long and “I enjoy Gow’s music.” /aɪ/, /ɔɪ/, /aʊ/, /juː/ for the diphthongs.
In the phonetics system I developed for Diinlang these would be represented:
Short: a, e, i, o, u, uh. “Dhat pen iz not mutsh guhd.”
Long: ah, ay, ee, or, oh, uu. “Pa(h) may wee orl go(h) tu(u)?”
Diphthong: iy, oy, ou (or au), yu “Iy enjoy Gou’s/Gau’s myusik”
This is only sixteen “vowel-type” sounds. This is less than my previous attempts at phonetics, although the above generally ignores combinations of vowel or diphthong with -r. In Diinlang the short e also represents schwa. This article shows systems with up to 24 vowel sounds for English.