You can make a big batch of shikanji masala and store it for up to a month in a glass container at room temperature. Whenever you want to serve nimbu shikanji, just add it to the glasses with the other ingredients.
You can also make a big pitcher of shikanjvi and refrigerate it for up to a day. If making it in advance, use water instead of soda water as the soda will fizz out with time and shikanji will not taste great.
Freeze the leftover in popsicle molds for a summer treat.