There are several reasons for him to master runes. His primary aim to learn the law of order will help him push forward the boundary of what's possible with runes. So the two of them work together.
Learning the law of order is not too far ahead of him. He just needs to become a transcendent then he will be able to comprehend the laws from the law fragments. But he can't break through without knowing what the sun god is marking on his body.
These two will take a lot of time. His practice with Rune making will take a few more years and his body needs time to develop better. A much stronger body will be able to handle heavier modifications to the law fragments and he will need a very strong body for what he plans.
Since there's no rush or immediate threat to him, he will just train calmly. Besides, Legion has a plan to handle the gods of the plane. All it needs is some time to kick off.
