You've been around the eBay block and seen a lot of changes. Seller protection has improved a lot over the years, as has Buyer protection. eBay protection for Business Sellers works best when you follow recommendations, such as having Buyers open a Return Case and only refunding after you receive the items back. For instance, eBay now gives you the option to return as little as 50% of the original payment in case of damage or misuse, which they didn't 18-years ago.
Your post reads as if Feedback is the main issue for you. You have nearly 1200 ratings, with only one Neutral and one Negative, so another Negative is pretty insignificant. What is more significant is how you're not following eBay policies when you Reply to such feedback.
Feedback policy | eBay
This is likely to be far more off-putting to potential buyers than the Negative itself.
Please don't let one bad buyer experience put you off of 18-years' of selling! Try treat these odd sales like water off a duck's back.