Thanks to Roger Clark's WS2812B LED example for providing code for the LED string. The number of LEDs in that example has been changed from 30 to 300. Uploading code to the Blue Pill is done easily using the Arduino IDE. You can't find instructions here if you don't have experience with the Blue Pill.
The power consumption strongly depends on the power supply voltage.
|Power supply voltage||Power supply current||Power|
The USB power bank goes into current protection mode when driving the LED string directly at 5V.
The power bank will be connected to a 3.3V buck converter. This will reduce the power consumption to 4.5W. The LEDs will not shine so bright, but they're still bright enough.
There probably are no cheaper and water-proof housing than junction boxes. They have cutouts which make it easier to mount connectors in the walls.