There are the IPA signs (International Phonetic Alphabet), that have special symbols as well, but not everyone understands/uses them.
In SpeakMoroccan, we tried to make things the most simple possible. Here is our adopted transcription.
For r, as it is different from the English one, we made it clear in those general indications, so that we don’t have to write r with a different symbol.
Now for Dariza, z doesn’t sound to be but a z (like in zebra) and writing Darija with z is just confusing. I think they avoid writing it with j, so that you don’t read it with a “dj” sound, Daridja. But again, in our general indications we made it clear that we use j like in “pleasure”. Besides, this is how you will find most Moroccans (mainly French speakers) transliterating.
I hope this answers your question. If not, then please just ask away! We are here to help :).
If there is an advice I can give you, then it’s that you try to learn the Arabic alphabet. It’s fun, and very rewarding!
Good luck, and be back with all those questions!