I have to think about the first conjecture, about spamming transactions to undo anonymity.
However, the second thing, about "tax declarations". Taxes as a function of economic relationship are extortion. Taxes to pay for public services are normal. So I don't even consider "declaring crypto transactions". If you want to declare it, do it with fiat. Use crypto when you don't want to declare it, or when you don't care at all about anonymity (use bitcoin).
I have a totally different proposal for taxes, if they need to exist. Note that as an anarchist, I think that the state shouldn't exist, and hence taxes are not a useful concept. But if a form of state has to exist and must be financed, I propose the following. The first is that you pay for services, like "police and law protection". Ownership you didn't declare (and didn't pay the "protection premium" on) will not be protected by law, but you are free to do so. If you don't declare your house, police will not come and kick out squatters: it is not legally recognised to be yours. If you declare it, you pay a permium as a function of its value.
The second is that nobody can own natural resources, including land. You can only lease land and natural resources. This essentially kills the real estate market as collectible, because you can't own a house for ever, given that it can only be yours as long as the leasing period of the land lasts, something you negotiated when making your offer for that piece of land. This is an old and wise American-Indian principle: land cannot be owned. Maybe you have a leasing for 50 years or so. Society ("the state") decides upon what parts of nature are open to bidding and what remain public ; people make different proposals (leasing price, period, usage restrictions....), and the state picks out the most lucrative one. When a leasing comes to an end (either because the contractor didn't pay, or because the period comes at the end), the state decides anew whether this piece of land (and every capital on it, like real estate) goes back into the public domain, or is leased out again. The former leaser can of course, bid again. This bidding can even be done 10 or 20 years before the end, so that the actual leaser knows whether to invest or not on it.
All the income of those leasings of land and natural resources is the "tax income" of the state. No more compulsory declarations, no more limitations of economic freedom because of taxes.