The difference between Taiyi Golden Immortal and Innate Dao Monarch can be summed up succinctly. One cultivates rules, and the other comprehends the Great Dao.
In the ancient era, practitioners aiming to attain the realm of Taiyi Golden Immortal needed to comprehend the Great Dao of Heaven and Earth. But after that era, the Great Dao became difficult to attain, and practitioners stopped trying to comprehend it, choosing instead to contemplate the rules of Heaven and Earth.
The Great Dao is the foundation of Heaven and Earth.
Rules are derived from the operation of the Great Dao.
Therefore, comprehending the rules is much easier than comprehending the Great Dao. Similarly, if one completely masters the Power of Rules, comprehending the Great Dao also becomes much easier.