WordPress image database migration issue

I am trying to migrate from a very old CMS to WordPress.
I have manged to export the database and map fields from the old database to a new WordPress Database so all the post text has been imported successfully.

I have got the file paths of the images from the old database and uploaded the images to a folder structure on the server.
I have managed to rebuild the image database so that the uploaded images show in the gallery.

The bit I am struggling with is linking the images to the posts.
I have added _thumbnail_id entries to wp_postmeta to link the images the posts but the images are not showing up as Featured Images as I’d expect.

I have also tried posting revision rows into wp_posts, this hasn’t helped.

If I upload an image manually via the front-end/gallery and attach it to the post, then the relevant records are created in the database, if I try to replicate this via SQL on the new database, the images are not attached to the posts.

What am I missing?
Thanks