It is natural for lower leaves to yellow and dieback. The brown spots on the foliage looks like septoria leaf spot, one of the two most common foliar fungal diseases of tomato. The other is early blight and management is the same.
Remove the lowest leaf stems to increase air circulation and remove infected leaves, and remove excess suckers. Keep plants well mulched to minimize soil splashing and water at the base of the plants.
You could spray spray with an organic fungicide (fixed copper) early next season. Fungicide treatments can help to protect new or un-infected foliage, they are not curatives. Remove infected plant material at the end of the season. Next year, select resistant cultivars when appropriate and increase spacing to ensure faster leaf drying. See our website for more information on septoria https://extension.umd.edu/hgic/topics/septoria-leaf-spot-vegetables
Lastly, next season only plant 1-2 tomatoes per Earthbox. If you have not fertilized your tomatoes, it would not hurt to do so now. Soluble fertilizers in liquid or powder form are very convenient to use and effective because the nutrients are immediately available. They are mixed with water and poured around plants according to label directions. Here is more on fertilizers in containers https://extension.umd.edu/hgic/topics/watering-and-fertilizing-containers