Manly should be concentrating on building up more depth in the strength, size, power factors while grooming a young up and coming half.
Roosters,Souths, Cowboys, Cronulla have all had success in playing structured power based games in varying degrees with offloads while not requiring two top class halves.
The advantage of having two top class halves is that you can grind wins and still have some attacking creativity and potency if one gets injured, but then you potentially lose depth in the forwards.
Foran and DCE struggled behind a very soft weak forward pack and that is two top notch halves---you have to maintain a balance.
I would be looking for a sharp tough young half with ball playing potential and using him at dummy half for short periods in his grooming season to keep Api fresh then in year three taking over from Green.(2019)
This way you gain 2-3 years at low to medium salary cap impact in the same way DCE came through the ranks but got annoyed with his pay scale during 2012 onwards.