Going through the robot posts that don't have a more specific tag, I found that my initial assessment of android having 3 sub-categories was somewhat flawed and that there's a 4th category among those posts, which I'll refer to as borderline androids. These are characters like Roboco-san and certain fan interpretations of Curly Brace (see post #5643970), who are largely human-like with only robot ears or mechanical limbs to distinguish them.
This line will at least be easier to draw by deciding whether we want to treat android like the animal_girl/_boy tags (2+ matching attributes to qualify) or like monster_girl/boy (must be largely mechanical). Using the former as the standard would draw the line at Roboco-san (mechanical legs + arms) and disqualify versions of Curly Brace that only have the robot ears, while the latter would draw the line at characters like Mega Man X and Aegis. Of course, this would except posts with visible internals through x-rays, damage, or other means, which would qualify for obvious reasons (examples: post #4997643, post #5487086, post #5769088)