Lowest carb fruit juice without artificial sweeteners?

Coconut water? Not juice, per se, but low carb with other goodness included.

Water. Sorry but fruit juices will inherently be higher carb. As you say the cordials all have artificial sweeteners in them now. Perhaps you could use fruit juice as cordial and water it right down?

What about infused water (berries, cucumber, mint etc)? or even a sangria mix without the booze?

How about using lemon or lime juice or segments to flavour water or carbonated water?

Cucumber slices and mint make a lovely drink. Often have a jug in the fridge in summer. Bung them in... done!

If infusing water is an option, I would add fresh ginger to the suggestions above.
It could be drunk as a hot tea or cold.
I usually grate ginger into a flask which I fill with boiling water and strain a 3rd of a mug out to top with hot water as a herbal tea. I suspect you could do something similar but allow it to cool in the fridge and top up with sparkling water to make kind of ginger beer equivalent.

If you are interested in hot drinks, there are lots of herbal infusions and most work cold. Make a very strong version in a mug with a teabag and about an inch of boiling water. Give it time to "stew". Then top up with food water and ice cubes. In the summer, I have one infusing next to the kettle and start a new one when I drink it.
I prefer fruit/berry infusions best for cold drinks but, following the ginger theme, lemon and ginger tea bags work well too.