Taking advantage of the ICN pricing, and looking to book the following DONE4:

ICN-HEL-xLHR-DOH-LHR-EZE-xSCL-BOG-SCL-xEZE-SYD-NOU-SYD-NRT-DEL-xHKG-ICN

Passes the MM validation tool without issue.

Stopovers:
Continent of origin (Asia): NRT and DEL (2 only)
Europe: HEL, DOH and LHR
Sth America: EZE, BOG, SCL
Pacific: SYD x2, NOU

Is there any issue with taking multiple stops in SYD? I thought you couldn't stop in the same city more than once, but couldn't see a reference to this in the rules.

Positioning flights are SYD-HKG-ICN vv with CX J class, using QF points.

Any pearls of wisdom? Itinerary is places I want to go, but always keen to maximise any status credits / miles where possible.

In this case there is no reference to such a rule because there isn't one.

(There is a rule regarding returning to POO*)

xONEx are limited by segments; total and continental.

(*POO: Point of Origin)

I had thought that you can only fly/transit in SYD/MEL only once? No?

Nonetheless, your itinerary is very similar to my DONE4, but in opposite direction.

Mine is for work but have two destinations that I am visiting as a side trip (thanks to OW rules )

ICN-NRT-DEL-NRT-BKK-SYD-EZE-LIM-SCL-LIM-EZE-LHR-DXB-LHR-GVA-HEL-ICN

but I may change to

ICN-NRT-DEL-NRT-[SYD-EZE-LIM-SCL-IPC-SCL-MAD]-GVA-LHR-DXB-LHR-HKG-ICN [in LA codeshare or LA flights]

6 segments flown on LA or LA codeshare flights count towards 6 segment required in business class for lower km requirement for Comodoro status with LA (which is their top tier membership). Plus, IPC is a very interesting place which is quite difficult to get to without DONEx.

No. You can transit or stopover in the same place as much as you like within the other general limits. For example, my current AONE3 ex-MNL includes two transits and one stopover in HKG.

Unless you originate in SYD or MEL you can trasit as much as your segment limits allow.

There are rules restricting the number of flights between ports such as PER/BME/DRW and SYD/MEL with a few exceptions for specific routings (e.g. -AKL-SYD-PER-SYD-SIN-LHR- is invalid, but -AKL-SYD-PER-SYD-JNB-LHR- is fine)

No problem on xONEx. *A RTW has the one stopover per city rule.