You are right that the hours were never the constraint. The thing that expands is attention, and it expands because the work has no boundary telling it when it is not happening.
What actually contains it:
Fixed slots, same times every week, and nothing outside them. Not ten hours somewhere in the week: Tuesday and Thursday evenings and Saturday morning, specifically. A floating budget is spent continuously; a scheduled one has edges.
Notifications off, entirely, outside those slots. This is the single biggest change and the one people resist. If a customer email can reach you at work, the business is running twenty-four hours a day in your head regardless of when you do the work.
One capture place. When something occurs to you at eleven at night, write it down and go to bed. The eleven o'clock work is almost never urgent; it is the discomfort of holding a thought. Writing it down discharges that.
A stated response time, on your site and in your replies. Within two working days is completely normal for a small business and it removes the sense that everything must be answered now.
An end time on the Saturday slot, with something after it that you cannot move. The morning becomes the whole day because nothing stops it.