Automatic transmission maintenance intervals are given in your owner’s manual. Follow them, and also make sure you realize that “severe” conditions are what most of us drive in city traffic. The general intervals are designed to refresh the conditioners and detergents in the oil, and replace most of the oil which has thickened from heat with new oil. This also, of course, removes sediment when the pan is removed and cleaned and the filter is replaced.
Assuming that 1/3 of the oil remains in the torque converter (I think that is a bit high, and some cars have a TC drain plug), the sediment is still trapped in the filter and pan. Routine service intervals will maintain a healthy level of good oil in the transmission and help clean any deposits that may have formed. Most transmissions that have been regularly serviced show no deposits for this reason.
Flushing is a money maker for shops. Either they advertise that they “flush” the transmission and simply service it with new oil and filter, or they use a strong detergent to dissolve residue, mainly in the valve body. But since they use something other than transmission oil to flush (if they do so), then 1/3 of your transmission might be filled with detergent, not oil, when the thing is buttoned up. Or if they flush with tranny fluid, what’s the point? If it really needs “flushing” due to a driveability problem, then make several short interval services, say 5,000 mile intervals or every oil change, and see if the fresh oil will loosen things up. If not, you probably need a valve body overhaul before things get really out of hand.
Since you don’t know the history of the car, I would simply follow normal service intervals unless you are having problems with the transmission. That should be more than enough to keep you out of trouble. If it ain’t broke …