I bought liquid sucralose on ebay - pure, uncut stuff dissolved in water. 2 to 3 drops per pint is all that's needed, it's potent stuff. You could carbonate with sugar, if you wanted to - 1-2 cups in a 5 gal batch should make it nice and spritzy, and it should ferment almost all the carbs completely out, with no risk of over carbonation. Or you could just keg it up and force carbonate.
'watery' sodas are usually under carbonated, I've found. It's not beer, it needs much more than 2 or 3 volumes. I have no idea what I run mine at; 60 PSI at 3 degrees C seems to work well, probably 6 or 7 volumes I'd guess, though still not *quite* as spritzy as commercial sodas. I don't know whether natural carbonation could approach this level before the yeast craps out, or the bottles start to explode. 60 PSI at 3 degrees would be a LOT more at 20 degrees. I'd definitely be using PET if I was using natural carbonation, as they can take the pressures more safely.