human weight scale (60-180kg) with gram resolution
Not possible, since you will not find any suitable scale that has a repeatability within so big measuring range. Scales are usualy made of steel with applied strain gauges, so the steel can't bend and return at the same position, it has a hysteresis. Top grade weighing scales have 6000 divisions in legal to trade application. So the resolution is 200kg/6000 = 0.033kg (of you buy 200kg C6 scale). This can be improoved using 24 bit ADC, but not for legal to trade applications, still there is no warranty that it will detect 1g change,  it seems impossible to me.