Is there a reason why you went with traces rather than pours? I count 7 signals per board, and they're all meant to be low impedance. You could even expose copper on the back of the board to be used as ad hoc heat sinks without spending any extra money. The weird little triangle loops on the back really stand out to me, even though you probably don't need to worry about the impacts of a loop in your circuit.
Thanks for the feedback. Everything I know about PCB design is self-taught. I'm a total beginner, so I'm sure there's a lot to improve!
There's only 2 routes per board, VCC and GND. I initially planned for SMD header pins that I didn't end up using, because soldering wire on bare pads was good enough. I also planned for 8 connection pads per PCB, but only used 2 to 4 in the final assembly. So yeah, lots of room for improvements in the PCB design! Definitely would need to spend some time on it for a higher power version 2.
My first few boards had exactly the same problem, it wasn't mentioned in any of the tutorials that I watched. All you need to do is add a fill region that encloses the whole board, and usually set it to GND.
You probably don't need to worry about EMI and EMC with DC, but if you want to make these dimmable you definitely want a ground pour "behind" any high frequency lines. You additionally don't need to worry about that if you aren't manufacturing, but it's still worth learning it the right way IMO. The sig/pwr-gnd-gnd-sig/pwr stack-up is well worth getting into the habit of (it has great EMI characteristics), and translates relatively easily to gnd-sig/pwr-sig/pwr-gnd stack-up once you've nailed down the design (which has amazing EMI characteristics).
Rick Hartley made these stack-ups popular (if he didn't outright invent them): https://youtu.be/52fxuRGifLU?si=8W1WXfJRHg3Oeep5