All that 5 and 6 and 9 crap is useless and those letter were and could be done with normal latin letters:
5 already has kh, 6 already has t, 9 already has d, don't know what people used for غ but you can write it with gh
The problem with 6 and 9 is that there are 2 very unrelated sounds for t and d in Arabic. Mixing them is a hallmark of non-native speakers tell signs and to an Arabic speaker’s ear sounds broken. It’s like when German speakers mix th and z sounds in English. It’s very jarring to the ear and requires you to parse the same word you’re reading twice in your mind to tell whether it was 6 or t (or 9 or d) especially that very similar words might exist like 6atreb and tarteb or trab and 6arb. I usually don’t use 6 and 9 either except when I want to emphasis the 6 or 9 sound in a word.
Kh and gh are problematic mainly because you’re relying on the special rule in Arabic that you can’t have 2 constants like k, g and h sounds after one another in a word. You’re relying on the wrongness of a kh or gh construct in Arabic to realize they must be standing for something else. I prefer them tbh, but I get the desire of some to have symbols for them.
غ Is sometimes 3’ but I have seen more gh than 3’ especially that 3’ mixes the academic romanization with the “Arabiz” romanization
My last name starts with a ق and in latin it's written with a K.
The tools were just to provide a way to write letters that do not have a corresponding letter in latin alphabet, it's not a way to provide all the nuances of pronunciation.
Also, afaik 9 is a ص in many countries, while 8 is a ق. But it is definitely my first time hearing of a 9 used as ض.