To visit the separate Dzong Kumbum cave complex you really need to spend the night in the valley. Make your way back to Ngadrak village and drive 6km to a parking lot, from where it's an hour's walk (gaining 400m) to the main cave entrance. There are four main caves here.
The main cave is much larger than Drak Yangdzong and winds for several hundred metres to a large Guru Rinpoche cave and shrine. A side shaft leads to an underground stream and pool. Exit the cave and climb up the hillside with the aid of ropes to the shallow but wide third cave before scrambling up to the narrow fourth cave, where you'll have to crawl on your belly to make much progress.