Usually a week of flushing is enough but 2 is what most of us do.
About the only thing lowering temps will do will make them turn purple but usually you do that earlier in the grow. DOn't know if it would make much of a change in 3 weeks. Might be worth a try. I've run them down to about 40 during lights off.
Lights off for a couple days at the end can help resin production. It stresses the plant and it produces more resin in times of stress.

The plants are looking very good.