Dyspnea can be caused by various medical conditions or a combination of things. Dyspnea was actually one of my first complaints after I was diagnosed with PMR and took Prednisone daily. My belief is that both PMR and Prednisone caused my problem. My problem was called dyspnea on exertion (DOE) except it didn't take very much exertion to make me short of breath.
https://www.ncbi.nlm.nih.gov/books/NBK499847/#:~:text=Go%20to:-,Pathophysiology,respiratory%20tract%20and%20chest%20wall.
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I'm surprised your cardiologist said to stop diltiazem. When my doctor discovered I was having blood pressures in excess of 200/100, three blood pressure medications were started. My cardiologist blamed prednisone for my hypertension and said my heart wasn't beating effectively because of an arrhythmia. I also developed left ventricular hypertrophy (LVH) from excessively high blood pressure. My cardiologist said everything combined caused my shortness of breath.

I never developed GERD when I took Prednisone. I did get a bad case of norovirus which precipitated a "cascade of blood related events." An ambulance was called because I was extremely short of breath. I was eventually admitted to the intensive care unit. I thought people over reacted because I insisted that being short of breath was normal for me.

I couldn't believe that I had multiple and extensive pulmonary embolisms throughout both lungs. I tried to negotiate my way out of the emergency room by saying I needed to go home to take a meatloaf out of the oven. I promised to call my primary care doctor and make an appointment about being short of breath.