Wow... slow down a bit.
I'm not convinced that is septoria:
https://www.ipm.iastate.edu/ipm/info/fil ... /12/27.jpg
Septoria would be circular spots on the undersides of leaves, starting with the oldest. The spots have dark margins and lighter centers.
Even if it is septoria, IME in my humid climate, tomatoes almost always develop some septoria before the end of the season. It is the most survivable of the tomato diseases. I just keep removing (and destroying, not in the compost) leaves that are showing symptoms. By late in the season, my plants may be pretty bare at the bottom, but they are still producing just fine.
Most of the common diseases of tomatoes are fungal and they can all be dealt with similarly: Keep your soil well mulched. Pull off the bottom leaves so that nothing touches the ground. Water the soil, don't water the leaves. Make sure you have plenty of air circulation (through spacing and pruning). Use fungicides preventatively.
Valley went straight to the big gun chemicals. I didn't look all those up, but mancozeb is a carbamate, that is it is in the same chemical class with Sevin and other carbamate insecticides. As such it can have toxic effects on humans and especially animals, including aquatic organisms if it makes its way into the water table. Organic fungicides would include Neem oil, Serenade, hydrogen peroxide, baking soda solution, diluted milk.
But again I'm not even convinced your tomato plants are diseased. They clearly are not "dying" and look overall pretty healthy. Tell us a little more. Are these plants indoors? What kind of potting mix are they in? How / how often have you been watering? How much sunlight are they getting? It is quite common for indoor tomatoes to show some purpling. You may be on the right track thinking about nutrient deficiencies. It looks like pretty small container, which means you have to work a lot harder on fertilizing. You have to water a small container pretty frequently, which flushes nutrients out of the soil mix. Where are you located?