I've never officially used charlieplexing before, so I wanted to find a good guide on creating the schematic. I followed the great example at https://stkwans.blogspot.com/2012/05/designing-large-charlieplex.html
I decided that 60 LEDs total would result in a good spacing for the size available. I am betting that some empty space between the LEDs will let the flex PCB twist evenly along the 120mm or so of strip.
The easiest way to lay out two double-sided strips of 30 LEDs was to create two arrays that each use 6 microcontroller pins. That way they both have the same connection footprint, and it gives some symmetry to the design. Half of the LEDs (even/odd) are on the back of the PCB.
I designed one half of the schematic in KiCad first, and once I confirmed that I could route it, I copy/pasted the schematic section and replicated the routing by hand.