So, some feedback.
As TomGeorge pointed out, I was feeding the Nano from two sources.
PaulRB also contributed with improvements. I now run the bluetooth from the same 5V as everything else.
Paul_B had strong feelings about using the Vin pin so I now use the 5V input.
About the fancy DC DC converter, I managed to short circuit it so it never made it to the final round.
I got a converter that can deliver up to 8 A. The 143 leds at full white should reach about 7.something A and I have a fuse at 7.5 and a software limit at 7.5.
When everything seemed to be ready to run it turned out that what worked fine with 7 leds got sluggish at 143 leds. After some googling it seemed the WS2812B:s was keeping the Arduino busy so it wouln't recive the bluetooth messages. I got APA102 leds instead and that did the trick.
Here's a video of the result of this project.
Thanks for all the help.