Oracle DBMS – Unusable Index on Primary Key

If you have the Partitioning Option then consider hash partitioning the two tables into 32, 64, or 128 etc partitions on the join key. Whether you use parallel or serial query, your memory consumption will be very much reduced.

The update [global] indexes option does not rebuild indexes after whatever operation you ran. It maintains indexes during the operation (see Updating Indexes Automatically), so that they remain available throughout.

After your first truncate, the index needs to be rebuilt - it can't be updated, it's already "broken" and unavailable. Oracle won't rebuild it automatically for the next operation you do on the table, even if you include the update clause.