Rather than a straight “customer service” percentage, I’d like to see a diploma type system… ie, customer service levels 1-5 (or whatever) that increase speed of service.
Customer service may be the only skill needed to do basic retail of goods such as gifts etc.
There should also been qualifications for food prep, barista, and other specialist skills. People with an interest should be cheaper to train.
Having level 1 in specific businesses would be a basic requirement; higher levels might open up more goods (ie; fancy coffees) or they might just speed up the service.
Some employees will refuse to learn things, some will learn things more quickly, any qualifications add to their “value” and the more underpaid they are relative to their qualifications, the less satisfied they would be.
It would be possible to have a super employee that was trained up in everything but to keep them happy would probably mean paying them so much that your business wouldn’t be profitable.