hvordan samler man værdier fra en pivot?

Det ser ud til, at den hurtigste måde at gøre det, du vil, er at ændre din counted2 CTE, så kolonnen TimesTested tage din logik i betragtning. Så det burde være:

counted2 as (
  SELECT
    client_id,
    CASE WHEN TimesTested >= 12 THEN 12 ELSE TimesTested END TimesTested,
    CAST(COUNT(*) AS varchar(30)) AS count,
    CAST(AVG(testfreq) as varchar(30)) as TestFreq,
    CAST(STDEV(TestFreq) as varchar(30)) Stdv
  FROM counted
  GROUP BY
    client_id,
    CASE WHEN TimesTested >= 12 THEN 12 ELSE TimesTested END
    )