How can I get data from rollup index by source index?

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser):
AWS OpenSearch 7.10

Describe the issue:
I need to configure an index management policy with follow high level specifications:
1 - Rollover and move to warm when index has 1 day age
2 - Rollup (grouping by an internal field event_id) and move to cold when index has 1 month age

The intention of rollup from step 2 is to be able to fetch what event_ids belongs to the indexes on cold, so we can move them back to warm programatically (with ISM APIs: Index State Management in Amazon OpenSearch Service - Amazon OpenSearch Service)

How can make that match between the stored data on the cold indexes (by event_id) and the index name?

Or what would be your best recommendation for this kind of use case?