[Gunter]

The 20deg off-set is not just for squealing; it's more for even pressure against the rotor when brakes are applied during running.
Squealing is not destructive, it's just a nuisance and treated with a combination of shims and special "glue" between pistons, shims, pads.
If the pistons are stuck, or frozen, they cannot retract and pull the glued-on pads away from the rotor. (See Bentley SC Manual 470-11)
Another issue that is often overlooked is centering the calipers in relation to the rotors. If the calipers are too much to one side, the new pads will make contact even before brakes are applied. Centering requires shims under the mounting bolts.
In this case, the MC/vacuum booster may be screwy; hard to tell from a distance. I would lift the car, remove the wheels, remove the new pads, install the old, thinner pads and exercise the pistons by pushing them back into the calipers (Channel-lock pliers work well) and out again with the pedal 3-4 times making sure that they move easy. Check/Set the 20deg required for ATE brakes (That should have been done during the rebuild) You can make a 20deg template from cardboard and use channel-locks to rotate the pistons. I have a pair with ground-off teeth just for that purpose so I can grab the outer edge of the piston without damaging it. If all of that doesn't work, concentrate on the MC/booster.