We are not noobs at tandeming.
Been doing it TWOgether sincde 1975.
Our first tandem way back in 1975 had Mafac (centerpulls) front and rear + an Atom (French) drum brake.
The drum was a PIA to keep adjusted and we suffered from regular spoke breakage on the rear wheel.
Our next four tandems (all customs), we skipped the drum brake part.
Used everything from Mafac cantis, rear U-brake, Matthauser brakes, V-brakes, Dura Ace . . . all strictly rim brakes.
We are a rather light duo, pilot @ 135 lbs, stoker @ 105 lbs.
We live in Arizona and have ridden plenty of mountains and high elevations (9,200+'). Have desended twisting 11 mile long 7% grades and steeper shorter ones.
Our method is to alternate braking . . . font/rear and to scrub speed before the next curve, if necessary. Creating turbulence by sitting upright helps also.
So far, so good for us.
As suggested, try using what you have. If you are uncomfortable descending then get an Arai drum. A disc is not intended to be drag brake.
Just our experience . . .
Pedal on . . .

Rudy and Kay/zonatandem