At only 140 lbs. you can easi;y go down to something smaller (110 liters) from the standpoint of flotation.
Since most modern 110 liter boards will bealmost as wide as your Rumba, your only "learning curve" is going to be fore and aft stability (where you stand on the board).
That should be pretty easy for you to learn, and then you'll be "on you way".
A Carve or Futura in the 110-125 liter range is what I would suggest.
That will take you quite a ways in furthering your skill level, and may be a board you want to keep for lighter wind days down the road.
Hope this helps,