The Grendel is an excellent choice and a step up if you plan on using it on deer over the 5.56 with its heavier bullets that offer a larger frontal area and increased energy. Accuracy is very good as well, although here are not many producers of it and a reliable supply may be a problem short of going online. The fact you don’t want to load your ammo or use a AR10 platform makes the choices you can go with quite limited, there are very few cartridges with a .378” rim diameter.
Handload and rounds like the 25x45 Sharps or 6x45 would make excellent alternative options. Both are fine on deer at average Michigan ranges and the accuracy of them is excellent, the 6x45 was one of the top favorites of benchrest shooters until the 6ppc came along and knock it off its perch. Components are plentiful and of high quality at reasonable prices.