I voted for [name]Seth[/name] [name]Jeremiah[/name]. [name]Seth[/name] is short and breathy, so I think it needs something long and heavy to weigh it down. [name]Seth[/name] [name]Jeremiah[/name] flowed the most naturally of all your choices.
(btw [name]Nicholas[/name] is also the first name of one of my brothers (I have 3)- not sure if that would be weird or not. But it does sound good with [name]Nicholas[/name]. )
I voted for [name]Seth[/name] [name]Nicholas[/name]-- it works really well and I don’t think it’s a problem that it’s your brother’s name. I like [name]Seth[/name] [name]Gabriel[/name] too.
[name]Seth[/name] [name]Gabriel[/name]!! Also I suggest [name]Seth[/name] [name]Daniel[/name](used to be on my list and [name]Daniel[/name] is a nice strong name to pair with the short, softer [name]Seth[/name]) Nice choices though it is hard to choose!