Hi Just did a XCVU13P with 2104 balls on 16 layers (8 singal layers 8 internal planes) .. with blind and micro via's (via pairs 1-2l, 1-3, 1-16, 14-16, 15-16)
is definitly doable..
with regards to the signaling, you need solid planes, but they do not need to GND.. the reference is not to ground, but to copper.. so you can reference the impedance from any potential.
good luckl