WS2812B strip glitches or flickers. What's the cause?
Voltage drop in the DC wiring could be a problem. Do you have a multimeter you can use to check the voltage at the DC power connections at both ends of the strip?
The wire being stranded isn't a problem. Not being thick enough would be a problem. If you can't check if you are getting a good 5 V at both ends of the stop just try using thicker wire for the power connections. Providing 5 V and ground connections (not data) to the far end of the strip can also help.
(Modifying your code to provide a test static led pattern will make voltage measurements easier.)