How to make an "all optional" schema from a schema in OpenAPI 3

Required properties of a base schema cannot be made optional. In other words, you can only add constraints but not remove them.

You need to do it the other way round - define a base schema with optional properties, then "inherit" from that schema and mark the properties in question as required.

components:
  schemas:
    # Schema for PATCH
    BaseDevice:
      type: object
      properties:
        id:
          type: string
          readOnly: true
        name:
          type: string
        owner:
          type: string

    # Schema for POST
    Device:
      allOf:
        - $ref: '#/components/schemas/BaseDevice'
      required:
        - id
        - name
        - owner