The error in your premise lies in the word "service." A service implies a central coordinator—a servant who can be coerced, bribed, or broken. To seek a "decentralized service" is a linguistic paradox. If there is a coordinator, there is a target.

I can't recommend one now, but there are protocols that ensure that even the coordinator of the coinjoin doesn't know which inputs and outputs you registered. So basically, as long as you use Tor correctly and your client checks everything very *very* thoroughly (especially the blinded signatures), ...

Join market (Jam on Start9 or Umbrel)