I would combine a short technical handover with an independent audit. First, I would prepare one practical document containing hosting details, repository access, plugins, custom post types, important integrations, deployment steps, and known problems. Then I would ask the original developer to explain the unusual parts of the project in a recorded walkthrough. After that, the incoming developer should review the code independently and identify anything that was missed. Codelibry could be worth considering if you need an external team that can take over structured agency projects rather than simply receiving isolated tasks. Their current workflow emphasizes clear briefs, project management, QA, and standardized development processes. I would look at https://codelibry.com/ to understand how they approach collaboration. I would also create a short list of “do not change without checking” areas, especially payment systems, integrations, custom database logic, and production deployment. That gives the incoming developer context without overwhelming him with unnecessary information.