I'd remove the spacers and change to a -17° stem. Sounds counter-intuitive but it works. Then put the saddle back to the forward marks on the rails. Then rotate your bars down until the ramps are horizontal or a bit below horizontal. The ramps are the bit of bar just behind the brifters. Sitting on the saddle and leaning as far forward as is comfortable, your wrists should lay flat on the ramps- rotate the bars until that's true. Then move your brifters however until the brake levers are vertical. In this position, with your forearms as close to horizontal as is comfortable, your elbows should be in front of your knees when your cranks are horizontal. That's the stem length you want. Doing all this will take the weight off your hands and reduce the pressure on your thumbs. You'll be able to use a grip where your wrists rest on your bars and take the weight while your thumbs go over the tops of your brifters and your hands sort of hang beside them, two fingers going under.