My ‘15 Denali XL with the 8 speed has 104K miles and still going strong. The 8 speed has a bad rep on this forum. The 10 speed is nice, it shifts well. I will say slow rolling around town on roads that are 25/35 mph speed limits I already get annoyed with how soon/often the 8 speed shifts and I could only imagine that being worse with the 10 speed.
Though GM put the 10 speed in the 2018+ Denali’s they didn’t just shun the 8 speed because it was so “terrible”.
The 8 speed is in all the C7 Vettes: Stingray, Z06, and ZR-1. As well as the ‘16-‘19 Cadillac CTS-V. Which IMO are all flag ship vehicles for top GM brands.
When purchasing used I’d place more value on maintenance records, vehicle history, and what vehicle you’re more comfortable with versus a few people’s bad experiences.
I also wouldn’t consider downgrading from the 6.2 to a 5.3 if the 6.2 is what you want.
Take that for what it’s worth. Good luck in your vehicle search!