CRO agencies work at scale. They have processes, templates, and teams. That works well for large enterprises with dedicated marketing operations teams who can manage an agency relationship. For most e-commerce stores, it's overkill and expensive overkill at that.
The problem with agencies is the handoff chain. You brief the account manager. The account manager briefs the strategist. The strategist directs the analyst. The analyst produces the report. The copywriter writes it up. By the time you receive findings, they've been filtered through four people who each understood your situation slightly differently. The result is often generic recommendations that could apply to any store.
With a solo expert, there's no handoff chain. You talk directly to the person who has 20 years of e-commerce UX experience, who has worked with ADIDAS, LEGO, KLM, Philips, Bol.com, and ANWB, and who is doing the analysis personally. The recommendations are specific to your store because the person making them actually knows your store.
The output is also different. An agency delivers a presentation designed to justify the agency's value. A solo expert delivers findings designed to be implemented. Developer-ready briefs. Prioritized by revenue impact. Written for your team, not for a pitch deck.