I own 2 of these (v8 models).. I think you did great with that truck.. Other than over paying for it! I love that blue.
Speakers dieing are kinda a known problem. Does yours have BOSE of Standard? I always just replace them with junkyard/eBay OEM speakers.
For the GMT360, the changeover date was 12/12/2003 for the 2004 model year. My Envoy is a 2004 model built on this date, and has both old and new..
The 2002 models have a few things I like over other years.
Yours might have a cabin air filter, was a 2002 model year only thing
Yours should have a oil level sensor, another 2002 model year only thing
Yours DOES NOT have a SAIS system (secondary air injection system). This was brought in 2004 at changeover.
The TCCM (Transfer Case Control Module) is a known issue for this year. Just remember to exercise the controller regularly. Can be replaced with any other year 2004+ and is no longer an issue.
The valvetrain noise is a common issue. Singer Sewing Machine.
Check the exhaust manifold. They are known for cracking. Its a inherent flaw on large displacement I6 engines.
Yearly cleaning of the throttle body is correct. While cleaning, disconnect the battery.
DO NOT USE THE OLM (oil life monitor). Do the oil changes religiously at 3000 miles. The VVT/CPAS (Camshaft Position Actuator Solenoid), and your wallet, will thank you!
Remember the oil pressure guage is FAKE. The pressure reading you see is a estimated oil pressure calculated by the ECM. It will read normal, even if there is only 3PSI.
Because of the design, and the way the engines run, if the thermostat goes bad, it tends to clog the Cat quickly. Be mindful of MIL lights.
The thermo-electric fan clutch is a known issue. Its advised when it goes bad to send the PCM out to get it tuned out, and replace the thermo-electric clutch with a normal thermal clutch from the 2008-2009 model years. I have this done on one of mine.
The 4x4 systems are always neglected, and the transfer case on these seem to be more sensitive to it. Change the Front/Rear/TC every 50k, and you wont have any issues.
The door handle pull being loose/broken is another common issue. Repair kits are available.
Blend door actuators are another common issue.
Edit2: Most common question on these trucks are about lifts and wheels. I cant help with lifts, but the wheels are specific to the GMT360. Isuzu Ascender, Oldsmobile Bravada, GMC Envoy, Buick Rainier, Chevy Trailblazer, Saab 9-7x. The Chevy SSR also works, but those are staggered wheels, so cant be used on a 4x4/AWD truck.
I know way more about these trucks than I should... Although I know a lot more about the V8 and XUV models.
has a lot of good info, and knowledgeable people.