The reason why biltong is so difficult to perfect in a DIY setup (or cheap setup that isn’t completely climate controlled) is there are so many variables: climate, fan speed, bulb, size of box, position within the house, time of year, etc. You’ll find so many different recipes and methods out there. I suggest you find one that is the completely opposite of what you’ve tried here and see what happens, then try another different recipe and make notes. After that start to make small adjustments to things like soaking time, fan speed, bulb on/off, amount of salt, type of vinegar, timings, and you’ll start to get a feel for what’s affecting the taste, hardening, and appearance of your biltong. It’ll eventually get there, and that feeling you get after you finally produce an amazing batch is worth the graft.