I'm not sure if hydro could compete on price any more, either. Batteries are so cheap.
At some point you get limited by fill/discharge rate, but the cost of storage in a big pumped hydro is still pretty cheap.
Lake Travis already has a power plant and is rarely every full for example. No one is going to start using pumped hydro there because there is no extra water to pump.
You don't actually need an excess of water, you need to be able to move enough between lakes though. You are not using any extra water from the system by adding pumps.
That's the real determining factor right, places with cheap big pumped hydro projects ideally already have big dams that are not full, so from that point it works. But then you need a reasonable amount of elevation between reasonably close lakes (or ideal new spots).
TVA (similar in size to ERCOT, mostly within Tennessee) is about to begin its second such facility, after Raccoon Mountain [0]. Run-of-the-river facilities exist (including two in TVA's jurisdiction), which are capable of pumping water "up" the dam (for later use during peak loads) — perhaps LCRA might explore the feasibility of this?
Regardless of how the energy is stored, it might also (eventually) make sense to join Eastern/Western interconnects (and thereby "store" the energy outside of Texas). But I know ego/"Texus"/pride mentality exists (having grown up in Austin), so I won't hold my breath on accepting Federal regulations...
[0] wikipedia.org/wiki/Raccoon_Mountain_Pumped-Storage_Plant