I'm still pretty certain that about the only way the OEM brakes might fall short is in the hills. Arai drums are discontinued but they were/are popular. They are always showing up used or as OEM on secondhand tandems. If I was the o.p. I would pursue no other line of modification but the acquisition (somehow) of a drag brake to supplement the OEM rim brakes. While looking for a used Arai drum I came across this:
https://www.sjscycles.co.uk/brakes/k...-arai/?geoc=US. Seller is first rate. Finding a hub with left hand threads is pretty easy. Fork swaps, frame modifications ... that way lies madness. FWIW.