Betta Fish: Please Help!!

Hi all, I don’t use Reddit at all so please bare with me cause I do not know what to do regarding this betta. I rescued this Betta this past summer as I work in student housing and someone moved out and abandoned it in a terrible small plastic tank. I’ve had this fish since about June and upgraded his tank dramatically, 10 gal, real plants, filter, heater. I’m not really a betta expert since I only took him because no one else wanted to and I felt terrible. It’s been going great with this fish until about two days ago when he started acting extremely lethargic, pale in color, and breathing rapidly. I asked ChatGPT what is wrong including pictures and I was told it may be ammonia burn to his gills, I don’t have test strips to see if it really was an ammonia spike. I did a 50% water change two days ago, and have been leaving his tank light off and not feeding him. Yesterday, I did a 25% water change, added the recommended amount of aquarium salt, and have still been keeping the lights off and not feeding him. I see him attempting to go to the surface to get air to only fall back down, so I lowered the water hight dramatically so he doesn’t have to swim up too far to get air. He’s still trying after these two days and I don’t want to give up on him. I’m too scared to use any pet store fish “medications” just because he looks so weak. This whole paragraph is a hot mess but any help and questions would be appreciated. First picture is from this morning.