You are correct that the CollateSubrequests parameter is only available in the Composite API, and it is not available in the Composite Graph API. The Composite Graph API does not guarantee the order of execution of subrequests.
However, you can still achieve your desired order of execution by using the composite-tree resource type in the Composite API. The composite-tree resource type is designed for hierarchical data structures, and it allows you to define a tree of related records to be created or updated in a single request.
Here's an example of how you could use the composite-tree resource type to create or update an order with multiple order lines in the correct order:
Create a new Order record using a POST request to the sobjects resource.
Create an array of subrequests for the order lines that you want to delete. Set the method parameter to DELETE, and include the ID of each order line that you want to delete. Add these subrequests to the records array of the Order record that you created in step 1.
Create a subrequest to update the order to change the currency.
Create an array of subrequests for the order lines that you want to insert. Set the method parameter to POST, and include the data for each order line that you want to insert. Add these subrequests to the records array of the Order record.
Create a final subrequest to update the order to lock it.
Here's an example JSON payload for a Composite API request that creates an order with two order lines, deletes one of the order lines, updates the order currency, re-inserts the order line, and updates the order to lock it:

{
    "compositeRequest": [
        {
            "method": "POST",
            "url": "/services/data/v52.0/sobjects/Order",
            "referenceId": "orderRef",
            "body": {
                "field1": "value1",
                "field2": "value2"
            },
            "children": [
                {
                    "method": "DELETE",
                    "url": "/services/data/v52.0/sobjects/OrderLine/001xxxxxxxxxxxxxxx",
                    "referenceId": "orderLine1"
                }
            ]
        },
        {
            "method": "PATCH",
            "url": "/services/data/v52.0/sobjects/Order/{!orderRef.Id}",
            "referenceId": "updateOrder",
            "body": {
                "field3": "newCurrency"
            },
            "children": [
                {
                    "method": "POST",
                    "url": "/services/data/v52.0/sobjects/OrderLine",
                    "referenceId": "orderLine2",
                    "body": {
                        "field4": "value4",
                        "field5": "value5",
                        "Order__c": "{!orderRef.Id}"
                    }
                }
            ]
        },
        {
            "method": "PATCH",
            "url": "/services/data/v52.0/sobjects/Order/{!orderRef.Id}",
            "referenceId": "lockOrder",
            "body": {
                "field6": "locked"
            }
        }
    ]
}