How to require a referenced block to be an inline block?

Just came across this whilst looking for something else and thought i'd leave a comment in case other people find this. You can get around the requirement for nasty nested entity/paragraph doom forms by making more use of sections, custom layouts and the layout options and layout builder restrictions modules.

So for example...

- Create a card block.
- Set up a custom layout for the Card Group section and some layout options (see examples below) so you can add a title and a summary. I used a custom options setting which was pretty simple so i could use stuff like rich text fields.
- You then use Layout Builder Restrictions to restrict usage of the Card block so it can only be placed in a Card Group section.

I find this significantly simpler from a dev and UX perspective and better than the typical 'all blocks in one column with extra helpings of massive nested forms in a small column' approach. :-)

THEME.layouts.yml

card_group:
  class: '\Drupal\layout_options\Plugin\Layout\LayoutOptions'
  label: 'Card group'
  category: 'Whatever'
  path: templates/layouts
  template: layout--card-group
  default_region: main
  icon_map:
    - [ card-a, card-b, card-c ]
    - [ card-d, card-e, card-f ]
    - [ card-g, card-h, card-i ]
  regions:
    main:
      label: Card content

THEME.layout_options.yml

layout_option_definitions:
  card_variant:
    title: Card variant
    description: The card variant applied to all cards in this section.
    plugin: layout_options_class_select
    multi: false
    default: default
    options:
      default: Default
      button: Button
      small: Small
    layout: true
    regions: false
    weight: 1

  section_heading:
    title: Section heading
    description: An optional heading added to the top of the section.
    plugin: MODULE_layout_options_setting_textfield
    default: ''
    layout: true
    regions: false
    weight: -50

  section_summary:
    title: Section summary
    description: An optional summary for the section.
    plugin: MODULE_layout_options_setting_formatted_text
    default: ''
    layout: true
    regions: false
    weight: -40

layout_options:

  card_group:
    section_heading: {}
    section_summary: {}
    card_variant: {}