Betta Fish: Ulcer, bacterial or fungal or tumor? Please help

Posted a few times, have not gotten any replies.

This dark spot formed just in 2 months and increased in size. I am reading the sunken center suggests this may be bacterial or fungal rather than purely benign tumor (iridophoromas). Scared to do salt treatment, reading about too many risks.

If it is bacterial and I do not treat it will likely ulcerate and burst? If it is a tumor treatment won’t hurt but won’t cure either? Would I use Kanaplex or Maracyn Two in hospital tank. Please note, his water is prestine, the nitrates have been climbing to 20 ppm, so been doing 50 % water changes and he seems happy with that. He’s been occupied at the bottom some looking for snails, appetite is still good. Please help, love him soo much! 💜