When creating or editing a collection, multiple images can be uploaded via the Media Library widget.
This is currently configured to use DropzoneJS, which is deprecated, but is an easy drop in and has some enhancements for bulk uploads.
After installing the DropzoneJS module, enable it for Media Library in the config settings.
Additional images can be added when viewing a collection.
Images are shown in a view, with Views Add Button for adding a new item.
The button is configured with a url parameter for the collection id.
Entity Prepopulate on the Image media uses this parameter to prefill a Collection reference field.
Corresponding References is configured to reference the new item from, given the collection reference on the image.
TODO: Can I hide the collection reference field when adding a new image?
TODO: Can I bulk import multiple images using a views add button?