let's combine a few ideas here
After the golems kill, let transposers take the items, the water flows away with any exp that isn't taken by the brains near the killing pen. Water passes by more brains (and why not a few more transposers, just in case of overflow and stuck states) and then at the end the moving water pushes the exp onto a wooden pressureplate.
If the pressureplate detects exp on it, the signal runs back and turns off the spawner. Figure you can even toss in a state cell or something so it's off for half an hour or an hour instead of turning on and off every 5 minutes.