I'd say you're right at the tipping point where implementing a booking system makes sense. The informal approach works great when you're doing 3-4 sessions a week because you can keep it all in your head and there aren't that many moving parts. But if you're planning to scale up to 6-8 sessions per week or more, you're going to start running into the same issues everyone else has described in this thread - double bookings, endless back-and-forth communication, clients getting frustrated with response times, and ultimately lost business because people move on to photographers who make it easier to book. Think about it from the client's perspective too. When someone is ready to book a photographer, they're usually comparing a few options. If one photographer has a simple booking link where they can see availability and confirm their session immediately, while another requires exchanging several messages over a couple days, which one do they choose? The booking process is part of your client experience, and making it smooth and professional sets the tone for everything that follows. You don't need anything complicated, just something that shows your availability and lets people claim a slot.