Import posts from another site

Import all posts from the old blog at https://oldsite.example into this site, preserve published dates, authors, categories, tags, and featured images. Run the import on a staging site first, check the first 20 imported posts for formatting, broken images, and category mapping, and then schedule the final import for Thursday after we confirm. If any images are hosted on the old domain, download and rehost them here and update the post HTML to point to the new URLs.

Before I run the full import from the old blog, make approval easy for the content lead. Produce a short report showing how many posts will change authors, which categories will be merged, list posts with missing featured images, and flag pages that contain hard-coded links to the old domain so the reviewer can sign off quickly or ask for changes.

I started importing the old site's XML export and it failed mid-import with a 512KB upload/timeout error. I do not control the old host and I cannot recreate the export easily. I fear partial content loss if I retry blindly. What is the most likely cause, and what step-by-step recovery approach will let me resume or complete the import without losing published dates, authors, or featured images?

Every migration we do ends with a long manual cleanup: duplicate categories, missing featured images, and dozens of links pointing to the old domain. Where are we regularly losing fidelity in migrations, and what single change in our migration checklist or tooling will cut the cleanup time by at least half?