You list 7 cities in 21 days - as noted by others that is a lot of packing and unpacking, even if the train trips were shorter. You lose half a day on arrival, another getting ready for departure, and 1/2 day per travel effort. You have thus lost at least 4.5 days already to travel of your 20 days on the ground.
Others have commented on the joys of each of your proposed cities and they are all correct so choosing will be your challenge... All of the locations you list are great, but maybe not all together on one trip.
As RS did in one of his travel skills shows, make your list and then prune it down to leave your top objectives with reasonable travel time losses. Also, although some people like night trains for long distance, I do not because my sleep is just not as good, and leaves me fuzzy the next day (fuzzier?).
Thus, We try very hard to stay at least 3 nights per location giving us 2 full days of "being there." We have also adopted the "what do we want to see in each place approach" (AM, PM, Eve) in picking cities and hotel locations within cities. We prefer 4 days per location so we can sit in cafes and have a leisurely coffee or Aperol Spritz occasionally. Of course, that means we miss some things, but mostly now we miss sitting in cafes in Europe while we are home.