When it comes to enterprise-level SEO, it's important to have the right team available. That team might be a dedicated enterprise SEO firm, or an internal one, depending on your needs.
Hiring an external SEO team can seem pricey up front, but what you see is what you get: you don't need to invest in recruitment, training, office space, or other additional costs. This makes budgeting much easier, and also means you can jump in straight away.