Your questions is:
"I wanted to back-up my datomic DB. I am familiar with the steps defined at https://docs.datomic.com/on-prem/backup.html but since the data size is huge (in TBs), I wanted to only backup a small preview of the entire DB, like one or two attribute values from every entity.
Also, if I had that backup present at some location, like S3 or something, is it possible to just copy only a certain part of the entire backup so that it becomes a small preview of the entire DB as described above?"
and no, it is not possible to select just a part of the database for backup. It’s all or nothing.
If you want to create a test-database, you would have to re-transact the data you want to keep into a new database. You can keep the timestamps (:db/txInstant) in the transactions, but the entity ids will be replaced with the entity ids the new database transactor assigns to them.