> freelance with the company for a trial period

Agreed. The risk here though is that with freelancing the expectations are higher, your impact needs to be felt in week one (or even day one in some situations) or else you'll loose the contract.

Regarding delivering value, you're right its very tough if you don't know the codebase. Although, if the codebase takes two weeks to learn before you can deliver a feature that means there's a problem with their onbarding. Maybe their documentation was horrible. Maybe their project setup could have been better automated. Often times addressing these issues are ways to instantly provide value to the team. Just make sure everyone knows what your working on.