您可以按日期的年和月对原始查询的结果进行分组：

SELECT EXTRACT(YEAR_MONTH FROM import_date) AS import_month
     , AVG(day_total) AS average_per_day
FROM (
    SELECT import_date
         , COUNT(`storage unit`) AS day_total
    FROM sdon
    GROUP BY import_date
) AS x
GROUP BY EXTRACT(YEAR_MONTH FROM import_date)

将202207这样的数字转换为2022-07是很简单的。

SELECT  AVG(Pallets) AS AveragePallets,
        DATE_FORMAT(import_date, '%b-%y') AS ImportMonth
    FROM sdon
    GROUP BY LEFT(import_date, 7)
    ORDER BY LEFT(import_date, 7);

（我认为不需要子查询。）