Delete all thumbnails & regenerate them?

Is there a reason you need to delete them first? You can use Regenerate Thumbnails if you need to generate new image sizes after switching themes or registering a new image size.

https://wordpress.org/plugins/regenerate-thumbnails/

This plugin also gives you an option to delete images related to unregistered sizes.

Note: the plugin is not tested with the last few releases of WordPress

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