honestly I think at your level of training, 3 full body workouts a week would be perfect. even twice a week would do you good.
I'm of the opinion that 5 day splits are great if you're a professional bodybuilder. those guys have done this so long, they do need 15-20 sets of bicep curls one day a week to get the muscle damaged enough for the body to build it back up bigger.
but you're talking about a guy who's been training for a decade, measures every gram of food he eats, and uses steroids. we are not at that 'level' of training.
do a little damage to the muscle, two or three times a week with time to recover in between sessions, and the body will be able to build your muscles up bigger, assuming your diet is good.
20 mins is a bit short for a workout though...my full body workouts take about 45-50 mins, and leave me spent, but only a little stiff the next day, not writhing in agony.