You also have to only buy things available purely on that system from people you are certain don’t collaborate with the government. If all of those assumptions aren’t true, your activity can be tracked with increasing precision and the authorities will see local transactions which you can’t explain otherwise and demand explanations.

The schemes Zcash and Monero use have never been tested at scale with a sophisticated adversary. The statistical methods used are probably fine for keeping your friends from knowing what you buy but it’s less clear that a government which can observe network traffic and receive KYC data from compliant merchants or compromised devices won’t be able to rule out some of the decoys over time, and since it’s a blockchain you’re gambling that this will hold true forever.