How do I create a specific, different than the $20off promo, coupon code for clients?

You can make note of your discount in your about me section or headline but there isn't a provision in the Rover platform to have a discount coupon or code to offer to your existing clients. Some sitters after receiving a booking request from an existing client will adjust the rate down to the old rate, some don't offer discounts. So it comes down to a personal decision to discount or not, best of luck