Even if you want to declare it now, in many jurisdictions you have already committed a tax crime, so you are in big trouble if you declare it.
Which jurisdictions require you to pay tax simply upon on acquiring bitcoin? Most that I see discussed on these forums only require tax to be paid when gains are realized, so when your bitcoin is sold, exchanged, spent, etc. Certainly that is the case in the US.
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The legislation of several countries obliges you to declare the mere possession of cryptocurrencies. The first one I found by googling is
Russia, but there are some in other European countries as well:
"The bill requires that citizens, individuals, and legal entities operating in the Russian Federation declare their cryptocurrency holdings... "Another one: "Spain’s government has reportedly approved a bill that requires cryptocurrency owners to disclose their crypto holdings..."
Even if I bought 10,000 BTC 10+ years ago for a dollar, I owe no tax on my now ~$400 million stack (nor do I even need to declare it) until I start to offload that bitcoin.
In countries with wealth tax, you should have declared you holdings years ago. Even if they don't have an explicit law requiring citizens to disclose their Bitcoin holdings, wealth tax does not usually exceed $1M by much.