Thanks everyone! Today I ended up doing a rescue mission since it really didn't look well. Did this both with my carambola, and also a Soursop I have that was practically dead. I was a bit aggressive, since it looked like it was going to die anyways so why not do a last ditch effort of something more intense
Took the tree out, removed soil and cleaned pot. There was def rotting roots. You could smell it and there were brown patches of soft roots. I cleaned the root ball with the hose, being quite aggressive and removing all the dead / rotting roots I could, just using my hands and water so I could really feel where the roots were soft and also be more gentle (I have a neuromuscular condition that affects my coordination so often when using tools I have even more trouble knowing how much force to exert and can't control it as precisely as I want, so I use my hands for most things). I removed as much of the soil from the roots as I could. I spent quite a lot of time just cleaning it up so there was as little rotting root and soil as I could remove. New soil, lots of draining, no compost, soil is just sand, coco coir, and a bit of azomite and rock phosphate. I dipped the roots in copper fungicide, a bit worried that will be too aggressive but I was confident that if I didn't, residual disease would still remain and just keep spreading. Potted them up, and watered them in with the liquid version of rock phosphate. I'm hoping they will pull through, I know I was very aggressive but so far I've found that many plants I do this with have fared better than ones I've tried to rescue with more gentle means. We will see! Fingers crossed!