In the wake of Coinbase and Circle closing accounts related to the usage of darknet markets, I've built a system that uses Shapeshift to send BTC for LTC, and then back from LTC to BTC. That hopefully will mix your bitcoin and avoid some linkage between the outputs.
I'm using it already and wanted to share it with you: https://shiftd.xyz/
Beware that it's a small tool I made for myself, I won't take any responsibility if anything goes wrong (make sure you try it out with a few dollars first to make sure it works correctly)
You do not need to use a tool like this or a tumbler. Just use an intermediate wallet when transferring from Circle or Coinbase. A small guide can be found in my post history, or if you look up "Plasma Shield" using the search bar.