Had this exact problem with an old beta a few years ago, definitely fin rot + possibly columnaris + maybe some fungal infection.
1: Add a sponge filter. A filter, while also not curing the disease, will help keep down levels of toxins and give a place for the bacteria to live in. Make sure it is specifically a sponge filter, as carbon media may remove medication.
2: New medication. While Melafix and Pimafix are good for treating their respective kinds of disesse, they are not as good for serious ailments. Maracyn and ICH-X would work much better.
3: More aquarium salt. If the infections is getting that serious, more aquarium salt will help out. Make sure not to add too much though.
I’m not an expert on fish, but these should help a bit!