https://cointelegraph.com/news/crypto-taxation-around-the-globe-what-do-regulations-look-like
Countries without tax on crypto
Malta
Portugal
Belarus
Panama
Puerto Rico
Singapore (individuals don't pay tax on profits from the crypto trading)
Slovenia (if you have less than 100 tradable days per year)
Germany (if you hold at least 1 year)
Liberland (very pro-bitcoin micro state)
Austria (if you hold at least 1 year)
As a private person, non professional, prfits from monetary metals and BTC are taxfree in Belgium also. Even no need to declare as income.
How are they going to know that you are holding crypto if you do not publicly disclose your addresses? It's only possible if someone is willingly paying their tax and one should do so.
KYC. At some point most people have to get on a gateway to convert fiat to crypto and, at least in the developed countries, those are increasingly requiring stringent identification. Additionally there are some smart people that are already creating tools to track wallet movement and they can use a variety of techniques to tie those to people and groups. Basically, it wouldn't be smart to count on avoiding the taxman.
The taxman is not interested in our crypto holdings, because they are not taxable. That is why we do not need to declare anything. The legal concept is that we buy gold or crypto with already income taxed fiat.
But once you sell it for fiat, they are interested to tax your profit. At least in my country.