Hello and welcome to FnF!

The main issues that I hear from you are the clamped fins, poor appetite, pale color and darting/rubbing in the tank. These suggect to me a parasitic infection, most likely Velvet (which isn't always visible to the naked eye), which I would treat with RidIch+ or Maracide.

I think that I see the reddish tumor-like thing on the gill cover that you are referring to but this is not a thyroid tumor (wrong location) and looks like a benign non-worrisome lesion that requires no treatment. Healthy gills under the cover are normally very red in color. Some gill material can stick out from under the cover from trauma and some sticks out from infection (parasitic or bacterial). Maybe clarification of what you are seeing or a better pic might help me here. Howevre, since she is breathing normally, I'm not too excited about there being a serious gill problem at this moment.

Your pH is perfect. The salt won't help your betta imo... she is a freshwater fish and does not require salt.

What, how much and how often were you feeding the betta - wondering about the SBD aspect of things.

It sounds like your other betta with the curled up bloody fins that died had fin rot with probable spread of bacterial infection into the bloodstream.

Glad you found us!

Bard