How can I make the "field_media_document_display field be filled with a 1?
In media-migration-step1.yml.twig I see:

So should be:
field_media_document/display: TRUE
field_media_document/target_id:
    plugin: media_file_copy
    move: FALSE
    reuse: TRUE
    path: 'public://grants/'

If still not, try:
field_media_document/display:
    plugin: default_value
    default_value: TRUE
field_media_document/target_id:
    plugin: media_file_copy
    move: FALSE
    reuse: TRUE
    path: 'public://grants/'