The best guards in the league are typically 6'4 or taller, even the athletic ones fit in this range. So at 6'2, I do question how much stronger he can get without losing the athletic ability that makes him a top prospect. Same for Chance Warmack, how much weight can he lose in order to become a better athlete, before he loses the strength that makes him a top prospect.
With Decastro I do see the ability to dominate in any scheme. Maybe I'm wrong, but I wouldn't judge him based off a rookie season where he injured his ACL and MCL in preseason. As far as his "averege athleticism" he compares to Steve Hutchinson pretty well
DeCastro
height 6'4.5
weight 316
40 yard 5.32
20 yard 3.03
10 yard 1.79
vertical 29.5
broad 8'2''
20 yard ss 4.56
3 cone 7.3
Hutchinson
height 6'5
weight 315
40 yard 5.15
20 yard 2.97
10 yard 1.77
vertical 33.5
broad 8'10''
20 yard ss 4.74
3 cone 7.82
DeCastro does need to get stronger. It was known before the draft, and he did go to a major college program, but for a rookie, it's not that big a deal.