Oh, I'd even argue against "amateur" and "pro" as being opposites.

Well, I do agree with you. They are more like a spectrum. And the words have different meaning depending on context. One can be a "professional" house painter (that is, I pay you money to paint my house) who acts in a non-professional (amateur?) way (leave my house a mess and don't do a good job).

In the example you cite, while we are "amateur" convention organizers, we often act very "professionally." (Sometimes more professionally than the nominal "professionals.")