You have certainly covered your bases well.

I was hoping for another kickdown switch.

The fluid and filter would almost certainly accomplish nothing if it was done 20K ago.

Take a look at the Bowden cable. It might be possible that this cable is hanging up. If the cable was pulled to a typical position when heavy pedal is used, the shift would be delayed until over 3000 rpm. This is normal.

Now, if the cable fails to return to its stop, due to internal friction, the transmission thinks that you have stomped on the pedal again and delays the shifts.

If the cable fails to solve the problem, it is likely a problem internal to the transmission. However, this is the least likely scenario.