I have something similar in mind ex MEL with, at this stage the first trip around October next year and the second one around March-April 2022.
I was going to ask why SIN as the "break" out of OWA but I see it was an essential stop. Any other reason?
Looking at something like this, assuming SIN is that break point. And I might get rid of the first stop in HKG
| MEL | HND |  |  |  |  | 5,048 mi | 
| HND | HKG |  |  |  |  | 1,805 mi | 
| HKG | SIN |  |  |  |  | 1,587 mi | 
| SIN | AMM |  |  |  |  | 4,884 mi | 
| AMM | LHR |  |  |  |  | 2,292 mi | 
| LHR | KEF |  |  |  |  | 1,180 mi | 
| KEF | LHR |  |  |  |  | 1,180 mi | 
| LHR | HKG |  |  |  |  | 5,994 mi | 
| HKG | MEL |  |  |  |  | 4,590 mi | 
|  |  |  |  |  |  | 28,561 mi | 
 
I would also like to maximise my use of availability in F so changes that would help would be appreciated. I think the flights to and from KEY might be the main challenge?
The essential stops for me are HND (or similar), AMM, LHR, and KEF.
And if I understand correctly, I can only book the first 3 flights now and the rest would have to wait till next year which I would need to do through the call centre and get them to add them to my existing booking number?