I had the same issue & considered exactly what you have written above. It’s a tough decision. From what I have researched my conclusion was the .223 is cheap to shoot, 6.5 Grendal has good ballistics for AR type magazine limitations and it’s an excellent deer round, the .458 Socom likely to drop most medium sized game out to 200 m.
Theoretically could a 6.5 Grendal with the right bullet be used to take down Black Bears if you hit the vitals when hunting? Idunno…. Something that I do have experience with is defense from a Bear, which is different than hunting due to bears behavior (pissed off), distance (close range) and point of impact (generally frontal vs side/vitals) are vastly different when confronted by a angry bruin, compared to hunting which you get to choose the ideal shot (heart/lungs). Some Black bears in BC are not small & can take a significant hit from a shot gun slug to put them down. Therefore, I think the 6.5 Grendal is a poor Bear defense choice compared to the .458 Socom average hard hitting 300 gr. Bullets. If you’re only going to take one and you’re okay with shorter distance shots…. get the .458 Socom upper with an adjustable gas block. Having said that I do not know of a factory 18.5" .458 Socom Barrel manufacturer.