You've touched on a couple things that aren't flaws or reasons to rebuild a transmission of shift computer.
Shifting at low RPMs while driving like a Grandma (I know what you mean by this, I drive the same way most of the time), you will likely be in 3rd gear before you get to 35 mph. If you hammer the gas off a light or are on it pretty aggressively, it will stay in gear longer. Driving easy makes them shift sooner. Being in 3rd by 2,200 RPM is not unusual by itself. If I'm out cruising, I'll be humming along at 55 mph and the RPMs will be 2,600 or so. And it will have gotten to 3rd long before that speed.
You didn't mention the kickdown microswitch at all. It likely isn't factoring into any of this, but if you hammer the gas and engage that extra microswitch, it will keep the car in the lower gear longer to give you some extra power. That's more or less the opposite of what you're mentioning though. You can test it pretty easily on an open stretch of road and put the pedal to the floor when you know you've already gotten to 3rd and you should feel the car shift back down to 2nd while you're gunning it.
Depending on what throttle adjustments you made prior, your sticking high idle/high RPM could be a few different things. Snapping the gas to get it to come back down might be telling you it is sticking mechanically. The throttle cable or spool that is. Your cable could be getting pinched in its sheath as it travels under the passenger side of the air filter housing. You can see easily if you remove the housing and notice if one of the clips is pinching or the sheath got worn/fretted away and the cable under it is rusted.
The other sticking high idle culprits can be a throttle spool spring that's not lubed well likewise with the spring on the butterfly valves lever arm. You might be a tiny bit out of adjustment with your idle microswitch settings (on the threaded posts which engage that switch), but that's not that prone to be solved with snapping the gas because your problem sounds like it is hanging up and needs an extra push to get it to go back to rest. Inside that throttle body are what they call decal springs and some have seen those get old and weak and not return the assembly to it's idle position.
Search on here for high idle threads to read more. We tried to start a useful technical photos thread and one of the ones in there is where the far end of the throttle cable is at full throttle or idle underneath on the shift computer. You could be out of adjustment there too for some of the problems you're seeing. It seems like your problems are no longer ATF level related or whatever after the drain and refills you've done.