One thing I'd want to understand is what the agency actually investigated and corrected, beyond producing service pages and backlinks.
You mentioned rebranding, which adds another variable. Did they document whether your business name, services, locations, and contact information were accurately represented across Google, your website, and other important listings?
I'd also want to compare the baseline before the rebrand with what happened afterward before assigning responsibility for the traffic decline.
I'm curious whether they ever showed you specific problems they'd discovered and verified were fixed, rather than simply reporting the work they'd completed.