Clear connection between SEO and small service business revenue

Freelance digital marketer has helped grow service businesses ( no e-commerce) using SEO.

For future projects. I would like to provide meaningful proof of SEO value (beyond site visits and page views) and use this for my pricing.

I don’t have enough space left in my head to become expert in analytics.

Are there companies that can set this up ( customer aquisitions, determining the value of leads, etc )?

Any suggestions? Thanks.