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[name_m]Dashiell[/name_m] is not my style but [name_m]Nash[/name_m] is starting to grow on me. So my vote is for [name_m]Nash[/name_m].
For [name_m]Dashiell[/name_m] I would probably go for more traditional names e.g
[name_m]Dashiell[/name_m] [name_m]Thomas[/name_m]
[name_m]Dashiell[/name_m] [name_m]Matthew[/name_m]
[name_m]Dashiell[/name_m] [name_m]William[/name_m]
If your style is not so traditional you could go for:
[name_m]Dashiell[/name_m] [name_m]Grayson[/name_m]
[name_m]Dashiell[/name_m] [name_u]River[/name_u]
[name_m]Dashiell[/name_m] [name_m]Dawson[/name_m]
[name_m]Dashiell[/name_m] [name_m]Jackson[/name_m]
([name_m]Son[/name_m] names seem to flow well to my ear)
[name_m]Dashiell[/name_m] is a favorite of mine and I like how it is a full name with a great nn. It seems more substantial to me than [name_m]Nash[/name_m]. I think [name_m]Dashiell[/name_m] would sound great with a 1 syl mn name like [name_m]Dashiell[/name_m] [name_u]James[/name_u] or a 3 syl like [name_m]Dashiell[/name_m] [name_m]Benjamin[/name_m].
[name_m]Dashiell[/name_m] [name_u]James[/name_u] was going to be my suggestion also! I like [name_m]Dashiell[/name_m] [name_u]Reed[/name_u], and [name_m]Dashiell[/name_m] [name_u]Gray[/name_u]. We also have a friend named [name_m]Daxton[/name_m] “[name_m]Dax[/name_m]” and I think it’s an awesome name that is much in line with your [name_m]Nash[/name_m], and [name_u]Dash[/name_u]!
I love [name_m]Nash[/name_m]. It seems like such a handsome name!
[name_u]LOVE[/name_u] [name_u]Dash[/name_u]! I put up with [name_m]Dashiell[/name_m] for a while since I love [name_u]Dash[/name_u] so much, but it eventually fell off since I’m not as fond of [name_m]Dashiell[/name_m] as I am of [name_u]Dash[/name_u]. [name_m]Dashiell[/name_m] [name_m]Oscar[/name_m] was my first idea, but I really liked [name_m]Dashiell[/name_m] [name_m]Lion[/name_m] and [name_m]Dashiell[/name_m] [name_u]Murray[/name_u] when I was considering it for my own list. 
Good luck!