Your thumb on the spool during the cast depends on how your reel is set up. If you have the mag force/brakes turned up high you should be able to get away with not having your thumb on the spool mid cast. The lower your mag force/brakes setting is the more thumb you’ll need. Your reel will tend to want to overrun in this case. Have your thumb on the spool when your lure hits the water to stop an overrun not matter how you have the magforce/breaks set. I wouldn’t have your thumb on the spool at all during the retrieve. You want it to be able to spin free during the retrieve.