Well, heartburn like symptom just below the sternum is usually due to gastric reflux (GERD) and acidity. So, if you have been tested negative for GERD, then probably the symptom is just due to acidity. There could be H pylori infection, and this can also mimic GERD. This can be diagnosed through blood test, carbon urea breath test and endoscopy. So, probably the endoscopy with biopsy ruled this out, but please do bring this up with your doctor.
Other than this, the symptoms could be inflammatory bowel conditions such as IBS, Crohn’s, celiac etc. Keep a food diary to note the food items which make you feel good, and those which worsen your symptoms. Liver and gallbladder disorders are probably ruled out, but do get your pancreatic function assessed.
Also you need to make some life style changes such as taking a late night snack, drinking cold milk, avoiding heavy meals and eating frequent small meals, avoiding too much of caffeine, tea, smoking, fried food and drinks both alcoholic and non alcoholic fuzzy ones, avoiding heavy exercises within 4 hours of a heavy meal, raising the head end of the bed by pillows to 30 degrees, and avoiding lying down for least 2 hours after food.
Even though no GERD is detected, and since you are improving with dexilant, a combination therapy of a proton pump inhibitor like dexilant can be tried with H-2 blocker such as ranitidine and antacid gel.
Please discuss all this with your doctor. I sincerely hope you will find this information useful. Hope you get well soon! Good Luck and take care!