That's a very solid case for contractors and almost nobody will have a meaningful problem with it. I've done similar things in the CAD / data management space.
A person with outside perspective, rare skills, experience, and most importantly, is the outside voice able to bring real options to the table along with production proven, time tested ways and means, is worth every penny.
The full time peeps should be taking you out for a lunch or two to network and gain some easy, high value mentoring, contacts...
I would in a second, and have had others invite me for this kind of thing. Some have remained in touch for years, and as they grew, have helped me as I did them.
If we had more of that going on, it would benefit nearly everyone.
But we don't.
The majority case boils down to head count quotas, and cost of labor factors. Often, these are both under specified, leaving managers to find contractors to fill gaps. The larger companies do this and it's chronic.