Hi Ben,
Thanks for reaching out. With regard to your concern about size of memberlist, the maximum limit size of this collection is 500,000 members only. The InternalApiError.UNEXPECTED_INTERNAL_API_ERROR could be due to a temporary issue with the API.Can you please try making the call again and let me know if you are still facing the issue? If so, please share the complete detailed request and response logs for the call so I can further investigate the issue. You can find information on enabling logging in the ReadMe of your client library. Please reply privately to author option while providing this information.
Thanks,
Nirmita Patel Google Ads API Team