Definitely not too laty to add syrup.
What I usually do is draw about half a glass with a wine theif and then add syrup in small amounts to the glass, then taste unti I get it where I like it.
If you have sediment in the carboy it will need to be racked, based on what I decided from my tasting/adjusting I add some syrup to the clean carboy before racking. After racking I will wait at least a few days before sampeling again. If at this point it still is not sweet enough I add more syrup, stir, wait a few more days, sample.......Once I decide I like it I usually wait at least a week and sample it one more time just to be sure.