What has worked wonders for me is to get a few Indian Almond leaves, pinch off a section a little larger than a quarter and toss it in. The tannins help fight off bacterial infection and fungus. I was a notorious betta killer in the past and my guy is over a year old now and looking fantastic!

I still have to clean his tank once every two or three weeks, but you can have your replacement water already conditioned with the leaf in there so it's not much of a change.

I have had your same problem with many of my late bettas, clamped fins, lifelessness etc after cleaning the bowl. Try the Indian Almond or even pesticide and fertilizer free (dead and brown) magnolia or oak leaves. They just benefit from the tannins.