It's always puzzling to me how people hear a player's demands and want him gone. Of course a player is going to make his demands publicly at the highest possible number. Robinson or his agent isn't going to come out and say, "we want 16-18Mil". In negotiations, you start high and work your way down to the real agreeable number.
Also, why tag and trade him? First of all, he would have to SIGN that tag before the draft in order for you to get a 2021 draft pick for him. Doesn't really seem all that likely. The last franchised WR to get traded was Jarvis Landry a few years ago, and the Dolphins got a 4th and a future 7th for him. Granted, Robinson is better but no guarantee you get much more than that with a deep WR draft and most teams being really tight to the cap. The teams that would be most likely to have cap room and be willing to send a pick for a #1 WR probably would only do so if they got Watson (NYJ, MIA) which means they wouldn't have any top picks anyway or Stafford (SF, Indy) which again means no #1 pick so probably wouldn't give up the 2nd round pick everyone wants.
Lastly, who gives a shit if he doesn't WANT the tag or want to play on the tag? Force him to play on the tag or sit out the season. If he gets to the regular season and still hasn't shown up, then you trade him for whatever. But I'm not worried about a disgruntled player. A disgruntled Robinson will still give effort and put up numbers, he was disgruntled this year and put up a near career year.