Thanks, Julius, that is really helpful!!

I have implemented the part of the method that I needed as follows:

/// Move FHIR resourceproperties in the followin order:
/// - resourceType
/// - id
/// - meta
/// - extension
ClassMethod FHIROrderResourceProperties(resource As %DynamicObject) As %DynamicObject
{
    #dim order as %DynamicArray = [ "resourceType", "id", "meta", "extension" ]
    #dim newObject as %DynamicObject = {}

    for index = 0:1:order.%Size() - 1
    {
        set element = order.%Get(index)
        set done(element) = 1

        if $EXTRACT(resource.%GetTypeOf(element), 1, 2) '= "un"
        {
            do newObject.%Set(element, resource.%Get(element)) 
        }
    }
    
    set iterator = resource.%GetIterator()

    while iterator.%GetNext(.element, .value)
    {
        if '$DATA(done(element))
        {
            do newObject.%Set(element, value)
        }
    }

    return newObject
}

Yes, although the Python code does deal with an actual collection object :)

/// Python reorder FHIR properties

ClassMethod PyFHIRResourceReOrder(resource As %String) As %String [ Language = python ]

{

    import json

    from collections import OrderedDict

    result = json.loads(resource, object_pairs_hook=OrderedDict)

    result.move_to_end("extension", last = False)

    result.move_to_end("meta", last = False)

    result.move_to_end("id", last = False)

    result.move_to_end("resourceType", last = False)

    return json.dumps(result)

}

This Python code is what I ended up with. It re-orders the properties "resourceType", "id", "meta" and "extension" to be at the start