I've been booking ex-JNB/CPT DONE4/5s for years, but I've never used QR at all, and am not sure I understand the rules regarding DOH on an ex-JNB/CPT ticket.

Previously, I've always had the first intercontinental flight be JNB-HKG or JNB-SYD, and the last LHR-JNB. If I instead start with JNB/CPT-DOH-xxx (perhaps Asia or Australia/Pacific), does hitting DOH mean I touched Europe and can't go to Europe at the end? E.g., could I do CPT-DOH-HKG at the start, and LHR-DOH-JNB at the end?

(I'm looking to avoid both QF and BA in J, after being increasingly frustrated with both over the years.)