Using two chained mixers mitigates most of this risk. Odds of having two bad mixers is far lower than just one.
To this end, I've written a Python 3 library and CLI for Privcoin and Bitmix. They can both be used separately, like
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privcoin mix --currency bitcoin --output_address 1aaaa...
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bitmix mix --currency bitcoin --output_address 1aaaa...
The improvement is using both, together.
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pip3 install doublemixer
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doublemixer mix --currency bitcoin --output_address 1aaaa...
This is completely client side, so no need to trust a third party. Just be sure to review the code of privcoin-python, bitmix-python, and doublemixer-python before use. Absolutely no warranties expressed or implied, no liability on the author's behalf, etc.
Source code for doublemixer
In action:

Just pay with the bitcoin: URI under the QR code, or the QR code itself. If you read the letter of guarantees you can figure out what's going on. Be mindful of the minimum and maximum amounts, for both services.