Adjectival / adverbial phrase

"Obama has been a wonderful President because, first, he is a handsome guy. Second, and more substantive, he is a wonder commander-in-chief and a brilliant economic policy planner."

Am I right to use "more substantive" rather than "more substantively" since it describes the notion that Obama is a wonder commander-in-chief and a brilliant economy policy planner? My understanding is that I should use an adjectival phrase to describe a noun and an adverbial phrase to describe a verb in a sentence construction like this.

I am not sure if this is relevant, but I remember seeing people use "first/second" and "firstly/secondly" interchangeably.

Thank you.