>> You need to carbonate straight water first before mixing.
Yes, that's what everyone has been doing with their Sodastream.
But in reality its a time issue. When you add the CO2 it will escape until it attaches with something. Once attached the bubbles become smaller and don't escape.
So you can add CO2 after, you just need to do it allowing the CO2 to attach.
This new low Salt soda water will pretty much discharge the moment you open the top unless you refrigerate it first. Like O3 the CO2 stays suspended longer in cold water.
You can then add the 4 tsp of salt one at a time, once the first one is dissolved the CO2 has that salt to hang onto and the other three go in pretty fast.