How can I obtain the last inserted row with INSERT ... SELECT in PostgreSQL?

I'm trying to batch-copy data in a specific order from one table to another in PostgreSQL 12-beta2. The table is *not* using sequences, but contains an composite unique Primary Key (user_id, object_id). In order to determine where to start for the next batch, I'd like to start off from the last inserted row (WHERE user_id >= last_user_id AND object_id > last_object_id). Starting off with this:

INSERT INTO dest_table
SELECT (user_id, object_id, object_type, colN, ...) 
FROM source_table 
ORDER BY user_id, colN, object_id  -- this is indexed
LIMIT 1000  -- batch size
RETURNING user_id, object_id;

... returns a table of 1000 tuples. I'd like to obtain the last inserted tuple from it. I've tried to do a SELECT around it, like this:

SELECT user_id, object_id FROM (
    INSERT INTO dest_table
    SELECT (user_id, object_id, object_type, colN, ...) 
    FROM source_table 
    ORDER BY user_id, colN, object_id  -- this is indexed
    LIMIT 1000  -- batch size
    RETURNING user_id, object_id
)
ORDER BY user_id DESC, colN DESC, object_id DESC
LIMIT 1
RETURNING user_id, object_id;

But that returns a syntax error: ERROR: syntax error at or near "INTO" LINE 2: INSERT INTO dest_table ^ I've also attempted RETURNING ... INTO variable [as described here](https://stackoverflow.com/questions/2944297/postgresql-function-for-last-inserted-id/2944481#comment64237933_2944335) , but that fails too: ERROR: syntax error at or near "INTO" LINE 23: RETURNING user_id, object_id INTO my_variable; ^ Do I need to create a function for this (e.g. plpgsql) or am I missing something obvious in plain SQL that let me do this? That would be highly favorable.