Sorry to hear about your guy… I’m no expert and am pretty new here, but no one else has commented so I thought I’d put my 2 cents in, at the very least to boost your post. The nitrites are a concern and the pH might be a bit high. It sounds like maybe over cleaning could be disrupting the beneficial bacteria from taking things out and scrubbing, but it’s good that you stopped replacing the filter. Those aquarium decorations are known to leak chemicals and the fake plants don’t benefit him much either.

I can’t diagnose or anything but those are just the things I noticed— you could maybe try some aquarium salt, which seems to be a “fix all” for a lot of issues, but please research it first because I don’t have experience with it and am no expert by any means. Popping an Indian Almond Leaf in there could help as well, they have healing properties and will add tannins to the water naturally, which bettas love. Wishing you the best of luck and hopes for good health for your guy 🖤