I just realized that we and the o.p. live in the same place. We ride the same roads. Our tandem club rides the same roads. 203mm are the biggest rotors any of the teams in our club (25 teams) have. The Captain of a tandem should have control of all the dynamic braking in my opinion. Of course you can't expect the Stoker to react quick enough to help with staying safe. Without buying new brakes, here is how the o.p. can improve their situation:
Make sure the calipers of the present brakes are properly set up (bled). A single air bubble in the hydraulic line can compromise the effectiveness of the brake. Bike shops are notorious for not being perfectionist enough. I don't trust them with my brakes.
Many tandems come with 'drag brakes'. Some use a 203mm rotor disc brake as a drag brake. That's what you may also have to do. Usually the drag brake is not hydraulic and you can use a friction shifter as the brake lever and apply it only enough to keep your speed from getting out of control and no more. It stays applied continuously all the way down the hill. It doesn't overheat because it isn't really working that hard.
A regular hand lever won't work because your hand will tire or you will sometimes grab harder than necessary and come to a stop when that wasn't intended. A sighted Stoker can see exactly how much or how little drag braking to apply. A blind Stoker cannot. A Captain with a blind Stoker has to have all the controls on their handlebars. You could possibly squeeze the drag brake lever with a rubber band. That's what I would do. Move the rim brake to the right hand lever and move the right hand lever to some other part of the handlebar where you can access it. It now becomes your drag brake. The rim brake becomes your main rear brake.
Or you could just use the two 203mm discs dynamically without any drag brake. Again, properly set up disc calipers gripping 203mm rotors are up to a 400lb. load. Me and mine (me 200, her 150, bike 50, loaded trailer 50 - 100) have no trouble stopping a downhill bomb towards a timed out green light on Cornell Blvd in Hillsboro just past the airport with just the front v-brake on our beat around town tandem when the rear brake wasn't working. When brakes don't stop you, something is wrong with them! There aren't 'better brakes' because the present ones don't work as good. If it's called a brake it better work. Period. I am offering to help sort this out and maybe our Stokers (both blind) can get acquainted. In any case, I hope you figure out how to get more satisfaction out of your tandem adventures. FWIW.