Guidance on using GROQ to exclude already added documents in a sub-service array

export const Service = defineType({
    name: 'service',
    title: 'Service',
    type: 'document',
    fields: [
        defineField({
            name: 'subServices',
            title: 'Sub Services',
            type: 'array',
            of: [
                {
                    type: 'reference',
                    to: [{ type: 'service' }],
                    options: {
                        // How do I exclude documents already added to the 'subServices' array?
                        filter: ({ document }) => {
                            return {
                                filter: '!(_id in subServices[]._ref)',
                            }
                        }
                    }
                }
            ],
        })
    ]
})

filter: ({ parent }) => {
  const existingEntries = parent
    .map(existingEntry => existingEntry._ref)
    .filter(Boolean);

  return {
    filter: `!(_id in $existingEntries) && !(_id in path('drafts.**'))`,
    params: {
      existingEntries,
    },
   };
},