Calculate SUM over a primary key and between dates

As mentioned, you just need to include some flag based on date of order (for that I assumed you have OrderDate column in Order table):

CASE WHEN MONTH(o.OrderDate) BETWEEN 1 AND 6 THEN 1 ELSE 2 END

so your query could look like this:

SELECT
    c.CustID,
    o.OrderID,
    v.HalfYear,
    SUM(ol.Qty * ol.Price) AS OrderTotal,
    AVG(SUM(ol.Qty * ol.Price)) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS AvgOrdersHalfYear,
    COUNT(*) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS CountOrdersHalfYear,
    SUM(SUM(ol.Qty * ol.Price)) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS SumOrdersHalfYear
FROM Customer c
LEFT JOIN [Order] o 
    ON o.CustID = c.CustID
LEFT JOIN OrderLine ol 
    ON ol.OrderID = o.OrderID
CROSS APPLY (SELECT CASE WHEN MONTH(o.OrderDate) BETWEEN 1 AND 6 THEN 1 ELSE 2 END AS HalfYear) AS v
GROUP BY
    c.CustID,
    o.OrderID,
    v.HalfYear;

SQL fiddle

If you're using SQL Server version 2025 or newer, you could use very useful function DATE_BUCKET:

SELECT
    c.CustID,
    o.OrderID,
    v.HalfYear,
    SUM(ol.Qty * ol.Price) AS OrderTotal,
    AVG(SUM(ol.Qty * ol.Price)) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS AvgOrdersHalfYear,
    COUNT(*) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS CountOrdersHalfYear,
    SUM(SUM(ol.Qty * ol.Price)) 
        OVER (PARTITION BY c.CustID, v.HalfYear
        ) AS SumOrdersHalfYear
FROM Customer c
LEFT JOIN [Order] o 
    ON o.CustID = c.CustID
LEFT JOIN OrderLine ol 
    ON ol.OrderID = o.OrderID
CROSS APPLY (SELECT DATE_BUCKET(month, 6, o.OrderDate) AS HalfYear) AS v
GROUP BY
    c.CustID,
    o.OrderID,
    v.HalfYear;

EDIT Here's updated query accordingly to updated question:

WITH OrderTotals AS (
    SELECT
        o.OrderID,
        o.CustID,
        v.HalfYear,
        SUM(ol.Qty * ol.Price) AS SUMOrder
    FROM [Order] o
    LEFT JOIN OrderLine ol ON ol.OrderID = o.OrderID
    CROSS APPLY (SELECT CASE WHEN MONTH(o.OrderDate) BETWEEN 1 AND 6 THEN 1 ELSE 2 END AS HalfYear) AS v
    -- or for Sql Server 2025 or newer
    -- CROSS APPLY (SELECT DATE_BUCKET(month, 6, o.OrderDate) AS HalfYear) AS v
    GROUP BY o.OrderID, o.CustID, v.HalfYear
),
HalfYearAggregates AS (
    SELECT
        CustID,
        HalfYear,
        AVG(SUMOrder) AS AvgOrdersHalfYear,
        COUNT(*) AS CountOrdersHalfYear,
        SUM(SUMOrder) AS SumOrdersHalfYear
    FROM OrderTotals
    GROUP BY CustID, HalfYear
)
SELECT
    ot.CustID,
    ot.OrderID,
    ot.SUMOrder,
    hy1.AvgOrdersHalfYear AS [AVGAllOrders LHY],
    hy1.CountOrdersHalfYear AS [CountOrders LHY],
    hy1.SumOrdersHalfYear AS [SumAllOrders LHY],
    ISNULL(hy2.AvgOrdersHalfYear, 0) AS [AVGAllOrders LHY2],
    ISNULL(hy2.CountOrdersHalfYear, 0) AS [CountOrders LHY2],
    ISNULL(hy2.SumOrdersHalfYear, 0) AS [SumAllOrders LHY2]
FROM OrderTotals ot
LEFT JOIN HalfYearAggregates hy1
    ON ot.CustID = hy1.CustID AND ot.HalfYear = hy1.HalfYear
LEFT JOIN HalfYearAggregates hy2
    ON ot.CustID = hy2.CustID AND ot.HalfYear <> hy2.HalfYear
ORDER BY ot.CustID, ot.OrderID;

Your example data is quite poor. Only 1 CustID and only one Year.
However, let's try to consider the task.
First, we aggregate the data in the OrderLine table and then use only these aggregates.
Then, we can skip the Customers table because the data from it is not used in the example, and the CustID column in the Orders table is sufficient for calculations. If necessary, you can connect this table in JOIN clause.

Year data calculated with PARTITION BY year(o.OrderDateTime),o.CustID

    SUM(OrderSum) OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Sum_Y,

Halfyear data calculated with conditional aggregation like

    SUM(case when month(o.OrderDateTime)<=6 then OrderSum end)
                     OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Sum_HY1,

Full query

select o.CustID, o.OrderId,
    year(o.OrderDateTime) as yy,
    o.OrderDateTime,
    -- count,SUM,AVG for year
    count(*) OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Count_Y,
    SUM(OrderSum) OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Sum_Y,
    AVG(OrderSum) OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS AVG_Y,

    case when month(o.OrderDateTime)<=6 then 1 else 2 end nHY,
    -- count,SUM,AVG for 1 halfyear
    SUM(case when month(o.OrderDateTime)<=6 then 1 else 0 end)
                     OVER(PARTITION BY year(o.OrderDateTime)) AS Count_HY1,
    SUM(case when month(o.OrderDateTime)<=6 then OrderSum end)
                     OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Sum_HY1,
    AVG(case when month(o.OrderDateTime)<=6 then OrderSum end)
                     OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS AVG_HY1,
    -- count,SUM,AVG for 2 halfyear
    SUM(case when month(o.OrderDateTime)>6  then 1 else 0 end)
                     OVER(PARTITION BY year(o.OrderDateTime)) AS Count_HY2,
    SUM(case when month(o.OrderDateTime)>6 then OrderSum end)
                     OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS Sum_HY2,
    AVG(case when month(o.OrderDateTime)>6 then OrderSum end)
                     OVER (PARTITION BY year(o.OrderDateTime),o.CustID) AS AVG_HY2
FROM  [Orders] o
LEFT JOIN
  (
  SELECT 
    ol1.OrderID,
    SUM(ol1.Qty * ol1.Price) AS OrderSum
  FROM  OrderLine ol1 
  GROUP BY ol1.OrderID
) ol ON ol.OrderID = o.OrderID

[fiddle](https://dbfiddle.uk/kEhQATc0)