You don't want to confuse bloat, constipation etc...with normal full tummy and treat needlessly. Usually with constipation and bloat-you will have other S/S along with the enlarged abdomen-like-more lethargic, hiding, clamp fins, buoyancy issues, stops eating and stops pooping, however, lack of poop isn't always reliable due to some fish will either eat it or destroy it.
IMO/E-fasting for 24-32h along with flaring to try and stimulate.
Feeding de-shelled pea pieces on occasion is not harmful to a Betta IME-However, you don't want to feed Pea as part of their staple diet due to poor absorption of nutrients in the short GI tract.
IMO/E-You don't want or need to soak the fish food-It is true that pellets will swell up in water, however, the stomach contents of a Betta isn't water-it is gastric acid and enzymes-this breaks down the food so the nutrients can be absorbed and waste expelled.
Fish foods that cause the Betta to bloat are usually high in grain/grain byproducts and it is the gases produced from the grains that cause the bloat-not the food itself per se. You may need to switch brand of food-It is best to feed a good quality varied diet in small frequent meals. Depending on the size of pellet you are feeding-it sounds like you might be underfeeding and poor nutrition can be a stress factor that can compromise the immune response.
In the 5.5gal unfiltered tank-how much and how often are the water changes, live plants, water temp, type of additives used, any tank mates and appetite.
How long have you had him-and has he ever been sick and/or treated-if so-details.
What kind of testing products used and what are your base-line test on the source water. What is the source water-well water, city supplied tap water. Do you have a water softening unit hooked up and if so, can you bypass it for the Betta if it is the type that uses salt.
Symptoms-lethargic with bloated tummy....No, buoyancy issues, clamp fins, hiding, scales sticking up-correct....
He is still eating, swimming okay and responding to you...correct...
Can you post a pic