Hmmmm, not so sure about not shifting by itself when in M mode. I took mine out yesterday to try the AFM V4/V8 thing and mine does downshift (at a stop for example, when I forgot to downshift) and up shift (it self protects from over revving, and can be overridden by some specific factors) in M mode. It's the "idiot proofing" GM has built into the 2024 C8. It pretty much won't let you do any harm, if you're not experienced with paddle shifters and tachometers.
As per page 186, 2024 OM
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BTW, M mode does defeat the V4 AFM thing, on the freeway on cruise control, flat road, no wind, where I would normally see it toggle between V4/V8. as others have suggested. Unless GM set the dash icon to lie about it. 
OP: Why not leave the car in auto everything mode until you hit 1500 miles or 2450 kms, so as to let it "learn" your driving habits during the break-in period, as described on page 176 of the 2024 OM. I know it's hard not to want to kick it around a bit, it's that kind of car, but you haven't cleared the 500 mile "no nonsense, no tom foolery" while driving it, break-in plateau.
