Therefore, common, or at least wide-spread acceptance of cryonics will save hundreds of millions of lives

You're missing a step here where you would argue that cryonics is likely to be successful.

According to givewell, the most efficient charities are sufficiently funded.

Kind of. You can't cheaply keep people from dying by spreading the meme that oral rehydration therapy works for cholera anymore because that's no longer the limiting factor, though at one point spreading the idea was one of the most effective things to do. Similarly the "eliminate smallpox" intervention was far more efficient than anything we have now, and was fully funded a while ago. The bound of what's "efficient" keeps rising. This doesn't mean that a GiveWell would say an additional donation to one of their top charities does little good. We're still talking about very large benefits.