Hellllooo!
When I want art to be 'fill in friendly' pixel-perfect lines I use Pencil Full Opacity And Color. W/ Smoothing set to preference [For me 65-85]
+ Round Pixelated.
And these are the sorts of lines that can be duplicated and are then fill-in bucket safe.
Outside of that ;; Round Pixelated on the Brush tool also looks a little less 'scattered' around the edges than other options; basically same settings.
Besides that for details, it can be useful to drop opacity or alter your color choice to complement the colors of the piece.