The main flaw I can think of is that insurance is a money-loser on average - otherwise the people selling it wouldn't make any money, so they wouldn't offer it at that price. I can't immediately find average ratios for life insurance, but typical payouts for medical insurance are 80% of premiums while the figure for property insurance is more like 50%.

In other words, the expected net cost of taking out the insurance policy, if you never decide to redirect it to cryonics or loved ones, is going to be somewhere between 20% and 50% of the premiums. Is that worth it for the extra flexibility it gives you? That's something only you can decide.

I sell life insurance as a big part of my day job, and you should be aware that buying a big life insurance policy for charity is the sort of thing life insurance underwriters tend to be a touch suspicious of. It can be done, but they'll ask for things like your donation history to that charity and why you suddenly feel the need to shower hundreds of thousands of dollars upon them. Insurance applications can be turned down for financial reasons too, not just medical.

It might be worth looking into which life insurance companies are friendly to cryonics.

Presumably, it would be more efficient to donate the cost of life insurance directly to the beneficiary. People often fund cryonics with insurance because they can't afford it otherwise, but if you could pay upfront without significantly damaging your quality of life, I'd expect that to be a good idea too.

Well it still costs money, so it will be a constant drain on resources as long as you have life insurance. Which I suppose is fine if the payoff to the charity is worth it as a terminal goal, but I personally feel very weird betting on my own death. (At least in the absence of children or other dependents).