Past simple would be possible in this sentence:
Person A: Did they eventually sign to a booking agency?
Person B: Yes, but after I stopped managing them.
After strongly defines the relationship between the two events, so it is not necessary to alter the tense to past perfect to define the order.
Before also strongly defines the relationship between the two events. You only need to use past perfect with words like when: without past perfect, the two events happened at the same time: with past perfect, one is pushed backwards before the other.
They signed to a booking agency when I stopped managing them - simultaneous
They signed to a booking agency when I had stopped managing them - had stopped is before signed
They had signed to a booking agency when I stopped managing them - had signed is before stopped
Note that I have kept the clauses in the same order in the three examples: normally the oldest clause would come first, so the clauses would be reversed in the second example.