"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.
You could say something like this: "I'm generally not able to disclose details about other projects. But I can tell you that their effect on my availability will be..."
That's only the case if you have literally no other projects. If you have even one other project, it's not the slightest bit misleading. All I recommended saying is that you don't disclose details about your other work. That statement implies nothing about the volume of the other work, except that it's non-zero.
One of the best skills you can develop as a freelancer is to read between the lines when doing client development. Some clients say they want a freelancer, but what they really want is a full-time, remote employee. They won't necessarily know that about themselves, and if they don't, they won't articulate it directly. So you need to get good at sussing things out.
Taking a pseudo-freelance, client-demands-full-time gig and trying to compress it into ~10 hours a week is a recipe for disaster. You won't please the client, you'll drive yourself insane, and you'll end up resenting the work.
The biggest favor you can do for yourself is to discriminate ahead of time. Determine which sorts of jobs, and which sorts of clients, fit with your model. Accept the ones that do; turn down the ones that don't. (Don't be afraid to refer someone who doesn't fit your model to a freelancer you know who is a better fit. It's just a good-karma thing.)
If I were to hire a contractor to do work for me, I would require exclusivity for the duration of the contract and not because I want a certain number of hours necessarily. Being a developer I know that the delivered value is not directly related to the number of hours, I really do and in fact I'm skeptical of the value delivered by people claiming to work long hours.
No, I would want exclusivity because focus is important and people (especially developers) cannot focus on multiple things at the same time, unless we're talking about things that the contractor is very specialized in, has done those things repeatedly and has a track record to show for it. For CRUD stuff for example, I wouldn't care. But for new, more difficult, more challenging problems that require focus and inspiration, then I would require exclusivity. On the types of problems that I currently work on, for example, I cannot focus on multiple things on the same day, or even the same week. Sometimes if I get distracted by conversations that are unrelated to what I'm working on, my whole day is fucked because then I have to remember the context I was in and I'm already tired and out of my zone and I can't get back anymore, ending up staring at the laptop with a stupid look on my face. I have days in which I notify everybody that I need to get work done and don't want to be interrupted, unless there's an emergency.
Sometimes I envy devops, or CRUD developers, or other types of freelancers that can rely on experience to place themselves on autopilot or that can deliver value in a short amount of time. For example devops get paid mostly for being on-call, which means they can get calls at 12 o'clock at night, but can also mean that they don't have much to do all day and thus can serve multiple clients at the same time.
And note that I'm not comparing the difficulty of the job here, but about the types of gigs on which you can rely on experience or on short amounts of focus to deliver value. Or who knows, maybe I don't have enough experience and some day I'll be able to place myself on autopilot.
That's the big distinction. You're imagining yourself as the client, but I'm imagining the thought process of clients such as I've actually had. In my personal experience, clients are almost always never developers. And they do conflate hours worked with value delivered. I'm sure there are exceptions to that rule, but for what it's worth, I've never encountered one.
If they're asking you "how much do you work," you can reply: "Considering my other commitments, I can give you 25 hours per week." They don't need to know what your other commitments are, and they'll probably be content to assume those commitments consist of other freelancing gigs.
My clients don't know any details about each other. They know I have multiple clients, but they don't know each other's identities, how many there are, or how much time I give to each.