Janet, I have seen one top-pruned umbrella pine, which was growing exactly as you'd expect - looking rather unbalanced, and with very extensive side growth, which would probably mean even more shade for your pool. Perhaps you might consider what is seen more commonly, and that's leaving the tops to continue growing but being pretty ruthless with the lower branches, aiming eventually for a tall clear trunk with a rather dainty 18th-century top parasol?