I have just upgraded to a Mac Studio, and run XP12 natively on the M processor (i.e. not Rosetta).
I can't remember whether the stec-55x is supposed to have altitude preselect working on the Entegras - because my last Intel iMac couldn't run these planes without flashing screens, so I hardly used them, and stuck with the G1000s.
Anyway, today, I found that altitude preselect (what I fed in on the Entegra PFD) was totally ignored by the autopilot. In fact, even the VS speeds were not kept to very accurately. Is this possibly a problem with the M processor? I had, earlier, tried the same plane in X-Plane 11 running in Rosetta, and the autopilot verged on being almost unusable for altitude control, and I gave up.
I, so far, have had very limited experience in X-Plane using Mac Silicon, so I don't know what works, and what doesn't! I get super fps, which is a big plus.