Zheng Fa's thoughts were actually quite simple:
The Yang God Technique and the Dharma Body Technique are used for enlightenment.
A somewhat inappropriate analogy:
If Zheng Fa were a computer system, these two are meant for system upgrades.
And the Tianhe Technique is equivalent to a system backup.
The problem is if there's an issue with the upgrade, then the Tianhe Technique would be needed to roll back the version.
As for why both the Yang God Technique and Dharma Body Technique are practiced together, it is actually Zheng Fa's bit of speculation:
Put bluntly, these two are both constructed based on the human body, are not inherently in conflict, and are somewhat complementary, which is what is termed Life and Spirit Dual Cultivation.
On the other hand, the Dharma Body Technique poses a unique risk to Zheng Fa—Zheng Fa's physical body is equivalent to Fusang Wood.
