Combine tsvector and timestamp in one index?
You could create a RUM index like this:
CREATE INDEX ON bar USING rum (
   to_tsvector('english', the_text) rum_tsvector_hash_ops,
   timestamp
);

And query like
SELECT * FROM foo
WHERE to_tsvector('english', the_text) @@ to_tsquery('english', ?)
  AND timestamp &lt; '2021-01-01 00:00:00'
ORDER BY timestamp DESC
LIMIT 1000;

You need a WHERE condition with timestamp for the RUM index.