SEO Freelance / With Google Map SEO

When offering SEO services, and it’s local, wouldn’t GBP be the best way to do it? No on page SEO, backlinks, etc. I heard all you need for GBP is reviews, and overall just a well setup GBP.

So with local businesses, should I mainly focus on GBP when freelancing? And not really any need for optimizing headers, meta data, on page/off page SEO?