Drop a straight vertical line from the outermost point of lower lip. Note your chin is recessive relative to that line. Do the same on all the 'after' morphs. The 8mm advance is the least recessive relative to that line.
I would choose the 8mm genio for the following reasons:
a: Relative to the basic aesthetic guideline (straight vertical line dropped from lower lip) for where the chin should be, it should be somewhat posterior to the line but not too much posterior to it. So the chin in the 8mm genio is best fit for that guide line.
b: Not only is your chin overly recessive but it also casts 'long. The sliding genio involves a cut made at an angle; a diagonal cut. When the chin is SLID along this diagonal cut not only is the chin advanced 'outward' or horizontally forward but it's also slid vertically upward which shortens it.
c: Because your lips are apart, you might have some lip incompetence; some strain when closing your lips. That type of strain is commonly associated with what I would call; MECHANICAL INEFFICIENCY. Something where it's just HARDER to move the lower lip 'UP' when the soft tissue (and lip moving mentalis muscles) are oriented backwards and downwards due to the bone orientation of the chin. Muscle use to close the lips becomes more mechanically efficient (easier) when the bone orientation moves outwards and upwards. The maximum of 8mm sliding genio would tend to minimize the lip strain.
d: With reference to your concern that a larger advancement would 'strain' your soft tissue in some way, that's not the case at all. On the contrary. It's the bone orientation of your chin, lack of support to the soft tissue that results in the type of muscle strain associated with the open lip posture you have. The 8mm advancement would tend towards minimizing strain, (not to mention MAXIMIZE aesthetics). So, I would disregard any 'concerns' about soft tissue ratios to bone movement and consider it a NON-ISSUE.
e: Possible STEP-OFFS also become a 'So what' (non) ISSUE in consideration of the reasons mentioned above where the 8mm advancement both maximizes aesthetics and minimizes lip strain. Easily addressed via filler to camouflage later down the line or filled in with some 'bone paste' during the genio.
f: With reference to suggestions of more CCW, the sliding genio (movement along an an upward diagonal cut) gives more of the LOOK of CCW. Sometimes the extent of CCW achieved via Lefort 1 and auto rotation of mandible is limited by the occlusion of teeth.
g: Increased labial mental fold is also a non issue or a MINOR trade-off given the benefits mentioned above. Like, SO WHAT???, you get a deeper fold there in exchange for much better soft tissue support, maximization of aesthetics and minimization of lip strain.
