I like [name_u]Corbin[/name_u] and don’t find it at all hillbilly. I also like [name_m]Garrett[/name_m] and have known a couple and never once heard anyone call them [name_u]Garry[/name_u]. You could use [name_m]Rhett[/name_m] as a nickname for [name_m]Garrett[/name_m] even. Though [name_m]Rhett[/name_m] is super handsome on it’s own, I do find it choppy as a full name with your last name. [name_u]Everett[/name_u] and [name_u]Barrett[/name_u] are other ways to get to [name_m]Rhett[/name_m]. Other ideas:
[name_m]Bennett[/name_m]
[name_u]Roscoe[/name_u]
[name_u]Orion[/name_u]
[name_u]Carson[/name_u]
I know you said you wanted a clearly masculine name but I think it could be fun to go with [name_u]Emerson[/name_u] regardless of gender. Maybe nickname [name_u]Sonny[/name_u]?