I am also in Amuni and was seriously struggling for orcs while I was upgrading my workshops. Took me forever. I had 3 amories upgraded to the maximum level and no other sources of orcs. To increase my production I looked at the Heroes Forge wonder, adding in a 4th armoury, and making crafted buildings like the the orc nest. Below is a summary of what I found:
My armouries make 1090 orcs per 1 day, which equates to about 52 orcs per tile per day. That doesn't include the 2430 population I'd need to build it.
The wonder at level 1 would make 489 orcs per 1 day, which equates to about 20 orcs per tile per day. Note that this depends on scouted provinces so your figures will be different. At about level 7 or 8 it becomes more efficient than one of my armouries, but that would take a lot of KP.
Amuni chapter orc nests make 360 orcs per 1 day, which equates to 45 orcs per tile per day. No resources or population needed.
Based on these figures I opted to start crafting as many orc nests as I could. I've gotten lucky with the recipes, so managed to add 2 to my city, and have a few more stockpiled.
For you I would recommend building a new armoury if you have the population to do it, and/or making as many orc nests as you can. I wouldn't have recommended building the wonder, but now you've got it certainly don't remove it. It'll grow in time.