When the husbandry is on point, snakes typically don't get sick. They have pretty banging immune systems. As far as getting sick from feeders goes, if you buy from a reputable dealer (MiceDirect, RodentPro, a breeder you trust, ect.) you're probably safe. Use common food sense for storing and unthawing them. If you feed live, parasites are a possibility but still pretty rare. Other than that, there's really nothing else you can do about food-related illness.

As for other illnesses, the ones you mentioned are the most common; scale rot, RI, burns, ect. Another is mouth infections. If your husbandry is good, those can still be caused by injury. For example, I've had baby garters get mouth infections after latching onto my finger during feeding time and hurting their teeth trying to yank part of my finger off. A mild mouth infection will be swollen and more red than the rest of the mouth; this can be treated by swabbing with a 1:5 solution of chlorhexidine and water. If you see swelling with a purple/brown color in the mouth that's probably too far gone and needs a vet. Also, any mouth infection that is causing a white 'cottage cheese' looking discharge should be seen by a vet. I've seen it treated at home before, but that's really gambling with the snake's safety IMO.

Mites are another common problem. Typically not fatal, but can spread around like wildfire. Best way to avoid that is to quarantine any new snakes for 90 days.

IMO the most important sign something is seriously wrong with a snake is lethargy/listlessness. It can be a symptom of anything from bacterial infection to organ failure, and can be kind of hard to spot, but I gauge it primarily by tongue flicking and response to a change in the environment (like your hand passing over or in front of the snake's head).