Update from yesterday: I'm hoping this may help others who would like to do the same as I am trying to do. I've been doing a lot of online searching and calls to the Buick store I worked at a few years ago. Here is what I am finding.
1. Bose system vs. non-Bose. I have the non-Bose system and wanted much better sound. Had I been able to get the Bose without going way up on packages (thus costs), I would have. Let's just say the factory speakers in my car were not very good. Although I'm not a huge Bose fan, it would have been much more satisfactory.
2. Here is what's different between the two systems (confirmed by the dealer service manager I used to work with): The 4 door speakers! Yes, that is the only difference between the two systems (at lease for my 2018 and the 2017).
3. The head unit, the two pillar tweeters, the amp and the tire-well subwoofer are the exact same between the Bose/non-Bose.
4. Bose code for radio is UQA and non-Bose is UQG. I've confirmed that the GM part numbers are the same on both for the subwoofer and amp. Unable to locate part numbers for head unit at this time.
Here is what I've done on my car thus far: I found a great plug n play (high quality) speakers from a company called Sound Matrix. The speakers are called Soumatrix Inspirit (for Opels and Chevrolet and Buick brands). There are also Extase (spelling??) but only seem to be for VW brand. I purchased the Soumatrix Inspirit speakers for all 4 doors of my car. Once shipped (purchased through Amazon), I installed them myself by removing the door panels. That was the worst part of it all! Once the door panels were removed, it was just one screw securing the speaker. Remove, unplug, plug in the Soumatrix speaker, replace in the door and put the screw in. All 4 doors were done (by me only) in about an hour. Well worth the money and time I spent. But now, about that Bose subwoofer in the spare tire well......
Doing more research, I know my car has the ANC (active noise cancellation) system and I personally WANT to keep that. I love a quiet car. However, it seems that the "subwoofer" in my spare is only being used for the ANC system and not for sound (as in music). That is my final goal: To activate my subwoofer! I'm hoping it will give me just a little more bass, thus allowing the Soumatrix speakers to handle the mids/highs better. Of course, any place I call wants to just sell me a bigger sub for the back which is not what I want.
To end, I'm at the point where the Buick service manager I know told me that (as far as he can figure) all I need to do is find a Buick technician who is willing to hook up the computer to my car. Once there, he said there is a setting menu and then a program menu. Once there, they should be able to check the box for the Bose system (UQA) instead of the other (UQG). By doing this, I should have my subwoofer activated! If I can get this done, I will post again for an update. BTW, I'm still using my factory 1" tweeters in the front pillars with my Soumatrix speakers and it sound fantastic. Although Sound Matrix suggests disconnecting them, I did my own thing and kept them on and glad I did.