What is the principle business code for a business consultant (not sales related) business organization / organizational development

I would put that under 541611, Administrative Management and General Management Consulting Services.  But you may find one you like better.  These codes are not always a perfect fit, so just arrive at something you find reasonable.

Here's a link to the NAICS searchable database:  https://www.naics.com/naics-search-results/.