Look, schizophrenia is an old disease. You really did not want to have schizophrenia during Middle Ages. At best, you would have been ordered to undergo exorcism. At worst, you would have been burned at the stake for possessing evil spirits. Compared to the current stigma still surrounding schizophrenia, we live in far better times. Conditions that now would be described as symptoms of schizophrenia were described without using the term schizophrenia. Just because Kraepelin first described it in the late 19th century and the term schizophrenia was coined in the early 20th century does not mean that sxhizophrenia originated at that time. But even we assume, counterfactually, that it did originate at that time, that would have been before the advent of modern pollutants and contaminants.

In antiquity and Middle Ages, quite clearly, modern pollutants and contaminants to which wheat is exposed simply did not exist. Even the wheat grown during those times differed from currently cultivated wheat. Yet, schizophrenia existed. Methods of growing wheat changed, wheat itself changed, but schizophrenia continued to exist. Does this fact now sway you away from believing that schizophrenia is linked to the modern methods of growing wheat?

Further, had this hypothesis been true, wheat-eliminating diets would have been overwhelmingly efficacious for patients with schizophrenia. After all, you remove the causative pathogen and the symptoms should go away, right? And yet, in studies, some small studies are for the benefit of GFD (gluten-free diet) and others show no beneficial effect. Clearly, this is an interesting avenue for research and more research is needed, but there is no overwhelmingly positive data in support of the hypothesis you favor. Since you like intellectual pursuits, here is a long and interesting article for you:

Use of a Gluten-Free Diet in Schizophrenia: A Systematic Review - PMC