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This is the best reasoned argument against Bitcoin, and crypto in general, that I've seen. I've long been bothered by some of the problems you raise, and on the other hand your idea of using crypto for one specific purpose is brilliant!
The one thing missing from your argument is the domain of complementary currencies, which are a "third class" of money, entirely unlike crypto, doing well the things that fiat money does poorly, and vice versa.
Some of these are specifically designed to support high velocity of money, a common objective of policymakers. They do so with negligible or zero transaction costs.
Properly used, complementary currencies can provide the benefits of a "portfolio effect" to the monies used by a nation, and greatly reduce boom/bust cycles and economic instability in general, as discussed in this paper:
(Aside: after writing the paper, I sent it with some trepidation to Bernard Lietaer, the foremost expert on complementary currencies. I hoped he would identify any errors on my part prior to publication. Instead, he informed me that I had essentially replicated his MIT master's thesis!)
