ESP32 + SN74AHCT125N, Wiring? Max. length data cable to SK6812?

Please bring some science to equation. Oscilloscope will be a proper tool to examine your signal and understand what is really happening with your data signal. Otherwise it’s so called guesstimate. For your knowledge using CAT5 cable for addressable LEDs is a bad practice. Unless you want use RS-485 for extending distance between board and your strip. In this case distance will be up to 1000ft.