If you get a good enough head and good enough all around speakers, you won't even really need a full sub.
The GM head unit is extremely dated at this point. I'd advise doing a full reinstall.
If you are on a tight budget and need to go in steps, I would go
Rear speakers->Front speakers->Head unit. In that order. Bearing in mind, with a new head unit you should cut out the Bose amp and do a speaker rewire.
*Not 100% sure here*
IIRC the wiring harness from the stereo to the amp is the same as the harness in a normal system (Bose only) so it doesn't need all of the dash ripped apart, you can find where they lead to the amp and continue the signal from there.
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