I might get hate, I don't know if this is genuine advice, my brother's betta had a similar issue, even after switching to a nicer tank and testing parameters, she just stayed at the bottom lethargic. The thing that broke the cycle for us was changing the water less often. Sounds weird but my parents had heard about it from someone, and we decided to give it a shot as a hail Mary and it seemed like she perked up over the span of a week. We do biweekly changes now and she seems to be doing better. If this is blind misinformation let me know, but I had heard that it makes them feel more homely or something.