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The issue of partial body morphs, and how DAZ has structured all this, is something else.

Most of M2's (V2) morphs are both Full and Partial body morphs. I was confused by what partial meant, when it was first introduced by Rob and Charles.
Full body means that the control is in the Body section. The slave morph might only be the left nipple, but it is still a FBM if the control in in Body.

Partial means that the morph doing the controlling is on an actual body part and is itself a real morph. (It can be a morph with no actual deltas, but it is on a body part.)

What DAZ did was to take those morphs that only affect a region of the body and have the FBM dial control one morph in a logical central location. e.g. breast shape morphs that do both breasts and controlled by a dial on chest. This chest dial may not actually alter the chest, but it affects the morphs on each collar.

These controlling PBM morphs, do not have the pbm- extension as a general rule.
These morphs can be controlled from both masters at the same time. This does not mess up a result, but you do not easily know the values. What I mean: breastshape8 in Body- set it to 0.5, go to chest - no number on the dial - breasts are larger anyway, turn the chest dial to 0.5 and the breasts are larger still, but the chest dial shows 0.5, the two collar morphs show zero.

Your idea about M1 is sound, but you are missing a strength of Poser. You do the dials on M2 until you get a shape that you like, right? OK, do a Spawn on every group that the morph affects. Copy this one morph for each group to M1, but go further. Delete all of the construction morphs from M1. You have the shape you want, why do you need them?

Leave the face morphs that make expressions, but delete the ones that make a character, your single morph is all you need to make Mike into Joe. The Face morphs are by far the worst size monsters.

M2 also has some very valuable Fix morphs. Copy these over.
When you get rid of FaceHeart and company, you will se a really big difference. Use M2 to generate shapes to Spawn, not to use in a scene.