Item level authorisation in Amplify (Gen 2)

I'm building a multi tenant application. Every user belongs to an organisation, in Cognito the organisation ID is stored in a custom attribute.

I'm using Amplify (Gen 2). How would one implement Item level authorisation based on a custom user attribute?

Group-based data access and Multi-user data access doesn't seem to be the solution.

Group-based data access: only a limited amount of groups are allowed per user pool (10k), I'm expecting much higher numbers.

Multi-user data access: Not efficient... due to the high number of users in an organisation. a.allow.multipleOwners().inField("organisation_id")

Amplify docs: https://docs.amplify.aws/gen2/build-a-backend/data/customize-authz/

Any help would be more than welcome.

Thanks!