Slow query and wrong row estimates for CTE

CREATE TABLE attempt_scores (
  id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  attempt_report_id bigint NOT NULL,
  score_value double precision NOT NULL,
  created_at timestamptz NOT NULL DEFAULT now(),
  attempt_report_updated_at timestamptz NOT NULL,
  student_id int NOT NULL,
  course_id int NOT NULL,
  assignment_id int NOT NULL,
  score_name citext NOT NULL CHECK (length(trim(score_name)) > 0),
  attempted_by citext NOT NULL CHECK (length(trim(attempted_by)) > 0),
  CONSTRAINT for_upsert UNIQUE (attempt_report_id, score_name)
);
CREATE INDEX ON attempt_scores (attempt_report_updated_at);
COMMENT ON TABLE attempt_scores IS
  $$The collection of assignment scores extracted from the LMS database.$$;
COMMENT ON COLUMN attempt_scores.attempt_report_id IS
  $$Each assignment attempt has an associated attempt report (attempt_reports)
where the scores of the attempt is recorded. This column is the pk value from
that table.$$;
COMMENT ON COLUMN attempt_scores.score_value IS $$The score's value.$$;
COMMENT ON COLUMN attempt_scores.created_at IS $$The timestamp the record was created.$$;
COMMENT ON COLUMN attempt_scores.student_id IS $$The student's ID.$$;
COMMENT ON COLUMN attempt_scores.course_id IS $$The course's primary key in the LMS database.$$;
COMMENT ON COLUMN attempt_scores.assignment_id IS $$The assignment's primary key in the LMS database.$$;
COMMENT ON COLUMN attempt_scores.score_name IS $$The source/name of the score.$$;
COMMENT ON COLUMN attempt_scores.attempted_by IS 'The users.role column in LMS';
COMMENT ON COLUMN attempt_scores.attempt_report_updated_at IS
 $$The timestamp value of attempt_reports.updated_at on the LMS side. We use it
to find new rows added since the last time we exported to Salesforce.$$;