How do I use one-on-ones to really develop my team?

You have turned one-on-ones into something real. Your people leave them feeling developed, and more than that, seen. That is rare and it is a genuine gift to give someone. Most people go years at work without a manager who truly notices them. Yours have you. The thing to guard, when you are this good at seeing others, is that it takes real energy, and it is easy to give so much attention outward that you run low yourself. Seeing people well is a form of care, and care needs refilling. Make sure someone is seeing you, too. So keep doing what you do, and add one small thing: after each one-on-one, note the single most important thing you learned about that person. Over time you will build a real map of who they are and where they are headed, which makes your good instinct even sharper. And find your own person who asks how you are doing. The one who sees everyone deserves to be seen.