[name_m]Hi[/name_m] there.
Here are my thoughts on the sets:
[name_u]August[/name_u] and [name_u]Sullivan[/name_u]-I really like [name_u]August[/name_u]. It’s handsome, down to earth, masculine yet gentle. The nns [name_m]Augie[/name_m] & [name_u]Gus[/name_u] are both super adorable. I don’t mind [name_u]Sullivan[/name_u]. It’s slightly different yet simple. [name_u]Sully[/name_u] is pretty cute, & especially as a brother to [name_m]Augie[/name_m] or [name_u]Gus[/name_u].
[name_m]Cassian[/name_m] and [name_m]Watson[/name_m]-This is probably the most unique set on the list. I don’t really love [name_m]Cassian[/name_m], but it’s kind of cool & certainly different. It seems a little more artsy next to the posher/geeky (cute geeky) [name_m]Watson[/name_m].
[name_m]Lachlan[/name_m] and [name_u]August[/name_u]-These are my two favorite names from your list. [name_m]Lachlan[/name_m] has a beautiful sound. It goes well w/[name_u]August[/name_u]. I think that [name_u]Sullivan[/name_u] goes better if you plan on using nns, but otherwise this is my favorite set by far.
[name_u]August[/name_u] and [name_m]Cassian[/name_m]-These go well enough. Maybe a [name_u]Gus[/name_u] & [name_m]Cas[/name_m] would be a little bit too close?
[name_u]Sullivan[/name_u] and [name_m]Watson[/name_m]-These pair fairly nicely. [name_m]Watson[/name_m] is not my personal favorite, but I like that it’s so quirky & different. It fits in the style w/[name_u]Sullivan[/name_u]. It doesn’t really have any great nns though, does it?
[name_m]Lachlan[/name_m] and [name_u]Sullivan[/name_u]-These are nice & Gaelic together. I like them both better w/[name_u]August[/name_u] though for some reason.
[name_u]August[/name_u] and [name_m]Watson[/name_m]-These are fine together too.
[name_m]Cassian[/name_m] and [name_m]Lachlan[/name_m]-These seem to be slightly different styles, but not so much so that I would say that they don’t fit.
Hopefully my rambling helps a bit. Great lists, so it’s hard to narrow down! 