Have all your team members work 30h and the "not pulling their weight" argument is gone.
Or if that is not sustainable/beneficial for other types of jobs than developer, only apply the 30h to your developer team. But do it consistent per team.
Another thought, I haven't seen this dynamic with part-time employees, so I don't think it applies to 30h employees either. Have you seen this with part-time employees?
Interestingly, part time is a different relationship than a compensation structure that is vague and discretionary. Part time people don't get high profile projects because when it's high profile, the point is to have every resource on it.
Regarding the CTO being incompetent in the other comment, the CTO is not incompetent for delegating authority to "get it done." Their priority is delivery and positioning the company. Do they seem weak? Sure, but how much political capital should a CTO spend on a part time jr. staff member vs. the sales organization?
Or if that is not sustainable/beneficial for other types of jobs than developer, only apply the 30h to your developer team. But do it consistent per team.
Another thought, I haven't seen this dynamic with part-time employees, so I don't think it applies to 30h employees either. Have you seen this with part-time employees?