I cut mine fairly hard back at work...back to about 6 foot tall. They put up loads of new grow...possibly another 6 foot of growth, and i find this young growth is far better in the autumn then older wood.

Don't know if this is the case with others but it works for me.

Rhus typhina doesn't always go red, often just yellow and then falls. If the leaves are shrivelling before they drop, drought might be an issue, but otherwise, the best autumn colours are achieved with plenty of frost about, particularly after a mellow autumn. It may be that yours, being on the edge of woodland, are sheltered from any frost there is around, so you get a long, slow decline to leaf drop rather than a sudden withdrawal of activity in the leaves (this is what causes the bright colours).