In my opinion it's an order of magnitude faster with AI.

That things come out perfectly working without need for QA and refinement cycles when writing the code manually seems very unlikely, and the case where coding it yourself is faster than explaining the task to the AI does practically not exist.

> as the project grows there's a risk of it becoming unmanageable

I suspect that what you said there is not based on actual experience but rather something you imagine. In my personal experience AI has shown no issues working efficiently on projects with even 500k LOC.