"Established" can be a tricky word when talking about plants. For some, it simply means "after their first year," but for others it can mean as long as three years.
Don't withhold water from your lilac if it appears to be undergoing stress.
I speak from experience here in the Bay Area with new gardeners and lavender plants. They want to grow lavender for the admirable qualities of the plant, including its drought tolerance. However
, that drought tolerance is only active after
the plant is "established."
Imagine the disappointment of these new gardeners when their lavender seedlings wither and die in the first summer (always rain-free except in very odd circumstances) here in northern California. "But it's drought tolerant!" they exclaim.
*sigh* "Yes, after it's established. Water it faithfully through its first summer and, sometimes, its second as well."
It sounds like a similar situation with the lilac in the Texas Hills Country.
Sunset Zone 17, USDA Zone 9