Because revenue does not create order on its own. Hard work gets you here, but structure is what lets you grow without holding the whole thing together in your head. The shift is from working in the business to working on it.
Yes, and more than most, because you are alone. A plan is your defense against bad clients and busywork. A big company can absorb distraction. A one-person business dies from it.
By the deliverable and the value it creates. Hourly pricing punishes you for getting good, since the faster you work the less you earn. Clients are buying the result, not your timesheet.
One strong stream can be fine. The danger is dependence you have not noticed, so that if it dried up tomorrow you would not have a business. Add related streams for the same clients, for stability, not for distraction.
Start by separating the decisions only you can make from the tasks you keep out of habit. Delegation works when you hand over the rulebook, not just the job. Order is what buys you the room to step back.