What explains esophageal pain + burn everytime right after taking oral ferrous gluconate with GERD, atrophic gastritis & neuroendocrine hyperplasia?

It's a huge pill. having an already irritated esophagus, likely with abnormal motility and then taking a huge pill will likely result in further irritation. Iron doesn't dissolve well. Furthermore, the entire point of iron in the body is to be able to easily add and subtract electrons. This also makes it inherently irritating as a chemical.