On a Thunder Tiger Pro 46, as the throttle on the transmitter is shut down, the engine takes a few seconds to go to idle. Could this be a result of the low end being too rich? Any thoughts on how to fix this problem?

The low idle screw is in the center of the throttle arm/carb barrel. Turn it 1/8 of a turn clockwise to lean it and try again. BUT, if it is rich coming off the high-end it's probably rich at the high-speed needle, too. Again: 1/8 turn clockwise and see if it helps.

Don't have a throttle delay set on the tx, by chance?

It could be a combination of being too rich on the low end AND not closing enough. So when you go to low throttle, the barrel goes to, say, 1/4 throttle, and then the engine slowly loads up and the RPM decrease.

Let it settle into an idle and pinch the fuel line. If it slowly revs up and takes several seconds before it dies, then the low end is too rich