Most legistlations consider Bitcoin transaction fees a sale for “a service”. Event if you are just moving sats between your wallets. So this would be considered a taxable event.
My biggest concern is about privacy. Because if you declare that transaction, and you get an inspection where whatever tax agency wants you to prove it, you will have to disclose the transaction id which is linked to past and future addresses. Which is a crazy bad thing in terms of privacy.
And even if you coinjoined you would have the same problem with coinjoin fees which, if you had to prove, would make the coinjoin useless in the first place (at least towards the tax agency).
Am I missing something? It would be unthinkable to have to declare capital losses every time one buys groceries with fiat, and being subject to presenting your groceries ticket to prove it.