Hi Dr. Acula,
definitely use the step down (buck) converter for any project using a car’s electrical system. Here’s a link as to why : https://forum.arduino.cc/index.php?topic=213399.msg1649634#msg1649634
and another forum discussing an Amazon and a mini little buck converter sold at Digikey:
https://community.particle.io/t/powering-boron-using-12v/46838/6
As far as coding the rings and strips separately, I haven’t tried with the pixeblaze. I do have the serial expander that allows you to connect up to 8 strips, but it treats them all as one big strip. If Pixelblaze allows you to set up 4 separate arrays or to treat on array as 4 (using a 4 starting positions and math that uses mod (% for short) to prevent drawing outside the array).
I like your idea of using a car’s panel!
I tapped into my car’s electricity by using a fuse “add a circuit” jumper cable, see here: https://www.autozone.com/electrical-and-lighting/fuse-and-accessories/bussmann-atm-add-a-circuit-fuse-tap/32416_0_0
With such short strips of LEDs you probably can get away with using 3.3v powered arduinos, which opens you up to using other boards, but if you ask me the Pixelblaze is a great combination of customizable patterns that has plenty of cool patterns already, and the ease of use of your phone or computer to change patterns. I have been able to do this with bluetooth low energy boards (Adafruit’s feather series) and with Particle Photons and a great app called Blynk, or Particle or Adafruit’s apps. Pixelblaze needs no app, and can run long strips (up to 240 per strip). I have 4x300 led strips connected using the Pixelblaze serial expander board. Only 240 out of the 300 are usable per strip, but a new board is coming out that can do a lot more. If you wanted to do long strips without the Pixelblaze, you would need an Arduino, or a more powerful 5V board, but they don’t have connectivity (BLE or wifi). I like the Adafruit Itsy Bitsy or the pjrc.com Teensy 3.5 for that. Pixelblaze is fit to task better, and I have to tap into that web side of things more, like that iOT post I just saw:
https://forum.electromage.com/t/hubitat-elevation-home-automation-integration/231
Have fun!
-Odor Decoder