[jackowayed]

If I were in your shoes, I would frame the rest of the time as professional development, which is likely a positive sign that might convince them to give you high rates that at times grow.

"I aim for about 25 hours/week of client work so I have plenty of time for professional development and personal projects where I can focus on learning instead of delivering the best results for my clients."

If phrased right, you can make it sound like you're already an expert at what you're doing for them, which is why you don't necessarily learn as much from it, so it's important to spend time honing your skills in new areas so you can expand your offerings.