Alternatives to Mixam for excellent quality?

I'll spare all the details, but I am a month into an absolutely miserable experience with Mixam. I went with them over other options because friends had had good experiences. In particular, I was worried about consistency across a mix of BW and color images where fidelity to both really matters. After reassurances and some conversations with reps, I went ahead with a small (100 copies) job to make sure the results were solid before a larger run.

Long and short of it is that the results were miserable. There were heavy magenta casts throughout the book on both color and mono images, and the color had a significant green shift.

I reached out, and the CSR moved me to a print resolution team. After uploading video of 40 books per their request, they agreed there was an issue and reluctantly agreed to provide a corrected press proof before the next run. [Full disclosure: I'm also a graphic designer and have extensive print production and pre-press experience, so I had checked and triple-checked all files.]

And the result was... worse. Color casts on BW had shifted to sepia (not as bad as before, but not what was provided or expected), and the color images visibly had blacks lifted with magenta casts still present.

u/Mixam's response? We did nothing wrong, cannot assure colors, and print variations are to be expected. Here's a link to our color variance page.

And the rep sent these images as examples of variation they consider subtle.

The printer I used in the past is out of business now, so I'm looking at my options. Thoughts? Suggestions? Better alternatives?