Ludicrous mode gives you eventual consistency guarantees. Also, there are no transactions in ludicrous mode. If you need linearizable reads and transactions, then you shouldn’t use this mode.
That said, you can still do queries and updates for existing data in Dgraph with ludicrous mode. It wouldn’t be transactional, but you can still do updates.
If you need to do a one-time load to bootstrap a new cluster, then bulk loader would be the fastest way to load the data.