You can say I bought the bitcoin at 50k but if you cant prove it, I assume they'll treat it as if you bought it for 0 dollars.
This is not how taxes work. Whatever you gain, whether that's bitcoin, gold, paintings, NFTs, cars, houses, cash etc., is considered estate. You don't need to buy bitcoin to get taxed or oppositely: If you don't buy bitcoin, but simply receive it, you're supposed to state it. How will they know? That's another story, and if you use centralized exchanges you can't get away with it.
So if like .74 btc vanished into XMR, trying to see if .74 or .73 shortly later "appeared" out of XMR. But afaik it's only speculative and there is no solid proof that was "you" who did that.
And so is bitcoin. If I use a mixer or coinjoin my outputs or simply transfer my coins from each address to another, there's no solid proof that I'm the owner of the new UTXOs.
They can only at best, if they found that, guess it might be you. But it could be anyone. This is why they hate privacy coins and want KYC for everything.
There's a difference here: Privacy can be invaded on-chain within certain limits, but KYC is for direct control. The ledger is public and chain analysis companies can investigate each user's activity, but that's a tiny friction of the power one can have with millions of users' data. Without the latter, the former can't harm much.