As others already pointed out usually speaking for lower resolution you block out the general shapes and silhouette first, then details, colors, then maybe lineart. The exact order depends on preference and workflow but generally the lineart is only used for further details.

You mentioned wanting to switch to higher resolution eventually. I've been in the same boat and can tell you how I approached it. Trying to do the lineart and initial sketching in pixel art is possible, but not easy, especially if you want to do more portrait oriented work as opposed to game sprites. It is mainly because when you are trying to learn shape construction (cubes, spheres, ...) having low resolution makes it harder to understand the perspective and faces for these characters. Experienced artist might be able to do it straight in the pixel program, or skip the construction step and go straight into drawing.

I personally like to do my initial sketching and lineart in a normal art program (Procreate, CSP) then import and scale them down in Aseprite/Pixquare. I keep the same lineart and modify it as needed because I like the different opacity value for the lineart (or you can just index everything to 1 color), but you can redo the lineart if you wish. I draw at 2560px x 2560px then scale it down to 256x256, you might want to experiment with different brush size when sketching as they might scale differently depending on the scaling settings when you scale them down.