Do you have any issue with diseases affecting tomatoes where you are at? For me, it’s not about keeping them from getting sick, just trying to manage it. This spring I made the mistake of transplanting a broken off part of one of my winter crop near some spring seedlings, which of course simply ensured that the new crop started off sick.
Have you tried a spray containing neem oil on your tomatoes? It is an organic spray that acts as an insecticide, fungicide and miticide. It works wonders for me. You can also mix some liquid calcium in every 2-3 weeks to assist in holding blooms and prevent blossom end rot.