you could draw the backgrounds on paper... either to use it as a sketchlayer
or to use it as line art as is.
if you make it relatively clean with black lines, then there's often tools in programs to turn "brightness to opacity" and you will have just the lineart as a layer.
even if that doesn't work, if you turn a black and white layer/ your scan/photo of your traditionally drawn lineart .. to multiply (works on anything with layer modes. Medibang mobile has it too e.g.) then you can also put a fresh layer below and use a bucket tool to color it in and the white of your scan should be transparent