First grow. Hoping for some advice along the way from some experienced growmies!

Of any of the plants that died could or did you dump them and look at the soil and roots, any clues there?

Yeah im going to concure with everyone else. Its a drainage issue causing your pH to spike and locking out your nutrients. Id suggest either buying fabric pots if you can get some and transplanting. Or drilling holes in the bottom, placing on a rack or something to allow drainage, and always water till runoff for a few seconds.