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As you attempt to downsized to HO or even N, you might want to consider using a "Raspberry Pi Zero W". It's footprint is about 1" x 3" (1/3 the size of an Arduino) and is only about twice as thick as the circuit board. It has on-board WiFi & Bluetooth; is very low power; and is $10 from Adafruit. I can easily see this fitting inside an HO boxcar and might even be squeezed into something in N gauge. That single18650 cell will be larger and more difficult to include than the Pi. Even if you can't find an N gauge car that it fits, with the right sized display, this would be a fabulous stationary sign/billboard for any layout. Just my 2¢ for what it's worth.
If you were to wire the 3 parallel strips to different power supplies which could be switched on and off independently, you could effectively have 3 different light levels: low, medium and high. While it would require more "bus" wiring, the resulting utility of 3 light levels might be worth the effort.