I feel you are in one camp or the other with coffee shop table etiquette, I wondered how many people are split each way.
YABU - it’s fine to sit at a table and allow one of your party to join the queue / effectively reserving the table.
YANBU - only grab a table once you have ordered.
In M&S cafe with two small children today so can’t “reserve a table” and leave them alone. While I was paying I saw the couple behind me swoop off and grab the only table free then looking a bit smug that I had to wait for one to come free. Some cafes have signs saying find a table first while others discourage it.
There is always an argument that sitting at an empty table actually slows down the over turn of tables and if people sat once they have ordered it could run smoother / faster. But then it’s a worry there isn’t a table and you have to wait.