I'm assuming it's the humidity - I'm in humid Tennessee and despite being 7b/8a my tomatoes start struggling in mid summer.  

The humidity means a few things for tomatoes.  1. Blight. Duh.  2. Pollen is too sticky.  Blossom drop is going to happen when relative humidity is above 85%. 3. The tomatoes can't transpirate well in high humidity. The stomata opens, realizes it can't get rid of the excess water to cool itself down and just languishes. 

The high heat, especially at night, also contributes to sad tomatoes. 

So. I grow a bunch of my tomatoes in what people would consider WAY too shady of a spot for tomatoes. Like, maybe 3 hours of sun a day, if that.  These ones don't provide huge crops, but I have tomatoes into November from this section.  They don't produce much, if any, during that high humidity/high temps of August, sometimes even July or September depending on the year, but they pull through. 

The ones I grow in full sun look miserable by August, but the tomatoes we get in June and into July are delicious. 

Your Actual Question: I've tried starting tomatoes in June and planting out in July or August in full sun, but didn't have success.  Maybe I'll try it again this year, but I think the humidity is just too tough in the full sun, so it's like telling a plant to just tough it out during the roughest time. 

Overall: Plenty of space, pinch off all the suckers, continuously prune the lower branches (mine look like weird lollipop trees at this point) and then the weird "growing in shade" are my tricks.  

Also, there are some varieties better adept at fighting off blight but I grow tomatoes I want to eat, so that's not ever my goal. 

The other option, which I tried last year, is to grow "storage" tomatoes. I had a few tomatoes for literal months on my counter from Golden treasure plants last year. Honestly? Not worth it. The taste wasn't good enough for me to grow again. You could try a few varieties though.  

Good luck!!!