18 sat/byte wasn't necessarily optimal, hasn't been for a while except for occassional spikes in the past few months.

Still, you should read the comments objectively and use it to improve your service... People who want to mix their coins want to do it for privacy. This also means that they want as much customisation and randomisation as possible. In your OP, you do note that any customising option is available for discussion, so that's what's happening.

Custom fees, custom delays, letter of guarantee... Very basic features mixers have. You would need those at minimum to try compete with the best.