The Ethiopia Gambella is a great SO.
Im not sure another setup will result in more prominent cocoa flavours. The corretto can produce these and other flavours very well. I use a lid on mine (an upside down lamington tray with a hole in the middle for the gun and slot on the side for the chaff).
To enhance the cocoa flavours, Id be keeping to your time frame, but making sure the ramp to FC was a gentle (no more than 10deg/min) from about 150 onwards.
Roasting to about CS 9/10 (into SC) will also enhance the cocoa but you need a bean that handles this well.
My favourite bean for cocoa flavours is the Mexican Tapachula El Tacana. You can take this one into a rolling SC and only get CS9. Andy mentions that the current Ugandan bean also displays cocoa flavours, but I havent tried it yet.
Resting time is also important for flavour development and leaving them for 5 days is usually best, sometimes much longer depending on the bean. Just roast more next time so you can start early and still have plenty into week 2. ;)
just my second crack