Mulled wine is, structurally, a gentle extraction problem. If you boil, it becomes harsh, cooked, and stewed, so the win is never to boil. Instead, warm gently until just steaming, then infuse below simmer, and sweeten with honey or sugar only at the end to taste. This gives you control, repeatability, and the ability to scale. You can make this in a saucepan, a Crock-Pot, or an induction pot on low hold. Flavor is stable for about 3 hours on warm.