hard to do when you can't proof where the btc came from. So many different sources, some stolen...
Cash and stable coins are not the same. Stable coins are like digital cash but the world is still far from a day when you can use stable coin or cryptocurrency (include CBDCs) to pay for all of your expenses in real life.

So let's obey the tax law regulation in your nation. When you actually cash out in fiat (not stable coin), pay tax per law, if you don't want to have jail time. 20 BTC has a big value today.

Even you will trade on real DEX, it does not mean you and your trades will be 100% anonymously. At least your trade partner knows such details (trade details, your identities)

Help: A list of LocalBitcoin alternatives (P2P marketplaces). Not all of P2P marketplaces are truly DEX.