Since tapping on the lid caused the trunk lid to fully open, then all you need to do is relieve the initial resistance
My guess would be to lube the hinges, or maybe even loosen the hinge bolts (slightly), enough to lessen the drag.
Otherwise, the torsion spring can be wound up a bit more for the tension you need...but based on your recent experience (that thing looks like it came off a garage door) I would stay away from the spring!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car