Not sure about combining, Max except that you won't know which it was that worked, if it works . .. .
That being said, I'd try the chelated iron first. Since they've had plenty of fert, Sue is right, it looks more like iron chlorosis to me now that we see the closer shot. The old leaves on the section with the flowers looks fine but all the newer leaves are yellow with green veins.
One more question, are they always in the direct sun? They really do better in the shade, especially when the weather's hot.
To answer your pH question, if you can't get a soil test done at your County Extension service you can get a soil test kit either online (I got one from Amazon) or at any nursery or big box store. A kit for one test costs about $4 but you can get one that will do 10 tests for about $12 if you want to test different
