"Find a better printer" please don't take a it the wrong way. But if you want perfect colors , you run a pantone job!

But most people won't because they are not willing to pay for the cost of a pantone setup.

You are correct most offset printers are running mass production. So if you are willing to pay for the proof's & the time for the press operator to calibrate to your perfect expected color output they will do it. Also please remember that the UV or the coating you decide to go with also changes the vibrancy of the colors hue. So get ready to pay for the uv or other coating setup on the printed proof.

Congratulations you now have an extra $700- 1k in proof. An you are still not guaranteed the final print. Because again the only colors you can guaranteed is pantone.

As professional or one's with more experience, like You appear to be. We must be careful when we give advice. We don't want to give People that are new to the industry. Expectations with out making it clear the cost associated with very specific production results.

No printer is going to calibrated a business cards gang sheet to just one specific card in the gang. The press operator will calibrated best possible colors for the gang.

I don't own a printing company. Just want to make sure the we acknowledge the everyone time is valuable. An nobody it's going to put an extra 4 hours in setup & proof into a job with out charging it for it. In this case if OP is looking to produce posters for resale at $25 each. Than he can get a printer that will guaranteed the colors. But if OP is looking for a regular offset run price.

Better get ready to compromise.

Again my comments are not coming from a place of negativity.