“Outsource the work.”
If it came down to it, what would - as you suggest - prevent the Christian artists from simply bringing the offending order to an accommodating shop down the street? In a rational world, nothing that I can see.
While reasonable, I suspect that if the Judge got wind of this, the Christians would be held in contempt. The Christians MUST be made to submit.
(”Submit”? Hmmm...isn’t that what the word “Islam” translates to? )
Christians have to see the writing on the wall, instead of going to court find businesses/people to outsource to. Avoid the courts.
Churches will need to stop performing State marriages in the future more than likely.
The only other options is to start going after Muslim/Jewish concerns in the same way.
Outsourcing would probably work actually. Just like out sourcing some complex part to a more qualified subcontractor. You probably should caveat in your fine print that “some or all aspects of any given job may be outsourced to affiliates better able to accommodate certain requests.”