Thank you for the tips. I have connected two cables at end of the strip to GND and +5V. Also connected additional GND pin from Pi Zero W to share GND.
Maybe it is a bit better but still not perfect. Made a video and two pictures how it looks. There is still flickering at end and when I enable solid white it gets more and more "yellow" color until it reaches end of strip. Still power issue? Could it be and issue with L corner elements?