I believe donating to the best charity is essentially correct, for the reason you state. You won't find much disagreement on Less Wrong or from GiveWell. Whether that's obvious or not is a matter of opinion, I suppose. Note that in GiveWell's latest top charity recommendations, they suggest splitting one's donation among the very best charities for contingent reasons not having to do with risk aversion.

If you had some kind of donor advised fund that could grow to produce an arbitrarily large amount of good given enough time, that would present a conundrum. It would be exactly the same conundrum as the following puzzle: Suppose you can say a number, and get that much money; which number do you say? In practice, however, our choices are limited. The rule against perpetuities prevents you from donating long after your lifetime; and opportunities to do good with your money may dry up faster than your money grows. Holden Karnofsky has some somewhat more practical considerations.