Econ relates to intelligence explosion dynamics, Scott Sumner appears to be a Correct Contrarian.

No, he doesn't. Edit: and I've found that his general reasoning to be poor in general. Some examples (which I can source later if anyone plans to update on this):

"Sumner, if what you're saying is true, shouldn't the Fed let anyone, including the average Joe, borrow from the Fed at 0%?" -- > "Yes."

Sumner: "Income" is a meaningless concept.
Critic: No, it's obviously vital to know how much you can spend before becoming unable to buy anything. And how would you value an enterprise but by discounting its income streams?
Sumner: If you want to know what a venture is worth, look at its stock price, not income.

I don't have any good ideas for how to do NGDP level targeting inside a cryptocurrency in a way that would automatically distinguish more widespread adoption from increased RGDP from somebody gaming the system.

Non-crypto, "real world" currencies have exactly the same problem of gaming the numbers to make NGDP artificially high. They're only worthwhile to pursue when NGDP specifically is targeted, rather than some "close enough" policy.