I'm rebuilding my website from scratch. Should I migrate all my old content?

No. A rebuild is the one cheap chance to leave the dead weight behind.

Migrate the pages that score well in your Page Inventory, ideally on the same URLs. Every old URL you don't keep must 301 redirect to the closest relevant new page, no exceptions, because those URLs carry years of accumulated trust and links, and AI search has already read them. That redirect map is your developer's single most important job on the project.

Launch with your core pages and your best existing ones, then let the content plan build and refresh everything else in priority order.