Treatment for Phonological Disorders

Hello everyone,
I have a question concerning treatment for a phonological disorder. There is a child who is 4;3 using multiple processes in their speech- final consonant deletion, backing, stopping, cluster simplification, stridency deletion plus a few more.

The child does not produce voiced or voiceless th, sh, ch, r, l, j, zh. It appears they have a moderate to severe disorder. They also do not use all other sounds in every position. What would be the best approach to use when treating this child in order to achieve the best result?

I know very limited information about the different approaches when targeting processes and I do not feel comfortable enough with it to pick a treatment option without some extra input. Thanks.