as it looks like there’s no way to specify that you want to add a NEW line when you’re in standard mode

This is how I do it

const so = new SalesOrder()
      const newline = so.item.addLine()
      newline.item = 4
      newline.quantity = 1
      so.save()

For standard mode, you need to know the count of the lines and increment that by 1 to add a new line. So if the current line count is 5, you use

objRecord.setSublistValue({
    sublistId: 'item',
    fieldId: 'item',
    line: 5,
    value: '1234'
});

using

insertLine()

at the end works as well, (that's what my code same does behind the scenes)