Options:
Seltzer water can be plain or flavored. I used raspberry and it was delightful.
Start out with half of the simple syrup and taste sangria. It may be sweet enough for you, depending on the wine you've chosen. Sometimes a 1/4 cup is enough, other times, I've needed the 1/2 cup of syrup.
If St. Germain is hard to find, swap it out for Cointreau or other liqueur. You can also omit entirely, but know that it will not taste the same.