Has anyone built their own Organisation service outside of Auth0

The limitations of 100k organizations, and the lack of a API to search/filter organisations has my considering building my own organizations store. In my system, I want a user to exist independently to the organization. They should be able to switch the organisation they are interacting with, which changes their permissions/roles, and org id in their jwt.

My understanding of Auth0’s organizations means that you get a lot of this out of the box. I am trying to work how difficult it will be to implement my own org selector (easy), that passes on the selected Organisation ID to login, and association of roles to a user inside of auth0, on a per org basis (for orgs that do not exist in auth0’s organisations).

Any pointers to the right terminology, or high level descriptions of the techniques to be used would be appreciated. Commentary on how painful this might be would also be welcome.