How do I make editorial calls when there's no clear right answer?

You have judgment now, and that is exactly why the gray areas feel worse, not better. When you knew less, you didn't notice how many calls have no correct answer. Now you do. The discomfort you feel is the discomfort of actually seeing the problem clearly. Subjective calls are not solved by finding the right answer. They are solved by making a defensible one and standing behind it. Ask what this particular work is trying to do, and choose the option that serves that aim. Two skilled editors could choose differently and both be right. Your job is not certainty. It is a reasoned choice you can explain. When you are stuck, write one sentence naming what the piece is for, then pick the option that best serves it. Commit to it and move on. The confidence you are waiting for does not come before the decision. It comes from making many decisions and watching them hold up.