Betta Fish: Little one has fin rot, can’t tell if it’s healing

Hello! This is my first ever reddit post, I just wanted some advice.

I got my little girl (Calypso) two weeks ago (I know she’s really a boy but I call her a girl). Since then she’s developed fin rot and these reddish splotches on her body. I’ve dosed her twice with the Blue Planet Tri-Sulfa tablets and done 3 50% water changes (conditioning with Seachem Prime) but I can’t tell if she’s healing or not. For background when I got her I also got a big piece of driftwood but it leached so much tannin the water looked like coffee. I’ve removed the driftwood and used Seachem Purigen to clear the tank. It’s still a little yellow but I quite like it. 20L tank, she has a filter, water is 26 degrees C, PH 6.5, Ammonia 0, Nitrates ~5 ppm, tankmates are 3 golden mystery snails. I’m also realising I am overfeeding her so I think I’ll fast her for a day then move to 4-6 pellets twice a day. I’ve never thought overfeeding was much of an issue since the snails hoover up the leftovers. She is still very active, eats her food, likes to hang out on the edge of the tank with me, tolerates water changes very well. Behaviour wise she seems fine. My concern is whether the fin rot is actually a fungal issue and maybe I need a new medicine.

Any comments and advice are very appreciated! I’ve learned almost everything I know from the good people on this subreddit. I’ve attached pictures from the day I got her and today. Thanks so much all