Warm the vegetable broth in a saucepan over low heat. Add the saffron threads and let them steep while you prepare the vegetables. This allows the broth to absorb the saffron's color and aroma.
Heat the olive oil in a large paella pan or wide skillet over medium heat.
Add the onion and bell pepper. Cook for 5 to 6 minutes until softened. Stir in the garlic and cook for about 30 seconds until fragrant.
Add the diced tomato, smoked paprika, sweet paprika, and thyme. Cook for another 4 to 5 minutes until the tomatoes break down into a rich sofrito.
Stir in the rice and cook for 1 to 2 minutes, coating every grain in the flavorful mixture.
Slowly pour in the warm saffron broth. Stir once to distribute the rice evenly, then do not stir again. This helps create the traditional lightly crisp bottom known as socarrat.
Arrange the zucchini and green beans over the rice. Simmer gently for 18 to 20 minutes, adjusting the heat as needed to maintain a gentle bubble.
Scatter the peas and butter beans over the top during the last 5 minutes of cooking.
Once most of the liquid has been absorbed and the rice is tender, increase the heat for about 1 minute if needed to develop a lightly caramelized bottom. Watch carefully to avoid burning.
Remove the pan from the heat, cover loosely with foil or a clean kitchen towel, and let the paella rest for 8 to 10 minutes.
Garnish with chopped parsley and serve with fresh lemon wedges.