Thanks all for your input! I ended up testing out three trucks:
2020 (used) Denali Ultimate with 5.3 + 10-speed
2021 AT4 with 3.0 Diesel + 10-speed
2021 SLT with 5.3 + 10-speed
The Denali didn't seem like it rode that much smoother with the ARC than the SLT did, and ride quality was a big look-for in this purchase; the AT4 was a bit rougher, which was to be expected. I didn't think the 3.0 Diesel was a smooth as the 5.3, though it definitely did have more snap under acceleration. I didn't find the diesel to lack power at passing speed on the highway as some here have noted, but it felt a bit sluggish under light acceleration at low speeds. In the end, because of my short commute I decided against the diesel, and on a feature-to-feature basis I couldn't justify the additional $10k for the Denali. I have a 2021 SLT in my driveway now, same color and mostly the same features as my 2016 SLT, however I can solidly say this new truck drives so calmly and easily, where the 2016 with the 8-speed was clunky and jumpy from the start. VERY pleased!