This assumes you don't have bose or onstar, and if any of this is confusing I'd give Sonic Electronix or Crutchfield a call to make sure you get the right stuff for your head unit. I would bridge channels 1-4 into just your front speakers, leave the rear stock and powered by the hu for now.
The above is ~500. The next dollars I would spend would be for this
Subwoofer box is 10 inches high 12 inches deep so check for clearance
Metra LC-GMRC-01 GM Class 2 Data Bus Interface https://www.amazon.com/dp/B001XOBU4C/ref=cm_sw_r_cp_api_i_Ig9XEb8R38HEE
thats is the link to the amazon. exact one i bought. it’s very easy to connect. i recommend getting an installation kit if you have a single din stereo, an antenna chord so you can switch from radio to aux, AND getting heat sink connectors or whatever they are called. the ones where you put the lighter up to it and it squeezes down on the wires. it’s a stronger connection than wire connectors that are twisted on. but either one is fine but it’s just a sense of security.
Eh, it's not horrible. Onstar in this car is obsolete so they could just use this (US price).