field.instance.default.yml

# field.instance.node.article.body.yml
default_value:
  -
    summary: ''
    value: "<p>default 1</p>\r\n"
    format: basic_html
    field_type: text_with_summary
  -
    summary: ''
    value: "<p>default 2</p>\r\n"
    format: basic_html
    field_type: text_with_summary

# Field instance defintion holds the top 'sequence' part.
# field.schema.yml
field.instance.*.*.*:
  type: mapping
  label: 'Field instance'
  mapping:
    default_value:
      type: sequence
      label: 'Default value'
      sequence:
        - type: field.[field_type].value

# Just type specific elements in data type defintion 
# text.scheam.yml
field.text_with_summary.value:
  type: mapping
  label: 'Default'
  mapping:
    summary:
      type: string
      label: 'Summary'
    value:
      type: text
      label: 'Body'
    format:
      type: string
      label: 'Text format'
    field_type:
      type: string
      label: 'Field type'

This way, we can have field.instance...*

field.instance.*.*.*:
  type: mapping
  label: 'Field instance'
  mapping:
    default_value:
      type: sequence
      label: 'Default value'
      sequence:
        - type: field.[field_type].value

and data type as

field.text_with_summary.value:
  type: mapping
  label: 'Default'
  mapping:
    summary:
      type: string
      label: 'Summary'
    value:
      type: text
      label: 'Body'
    format:
      type: string
      label: 'Text format'
    field_type:
      type: string
      label: 'Field type'