[MadSudaca]

The first TTC manuscripts date to around 300 b.c. and there’s evidence that some of the poems have a rhythmic structure, which could mean some verses were transmitted orally prior to that. So there’s a good chance that it was known prior to 300 b.c. between that time and when legalism crumbled, there were other renowned Taoist sages, such as zhuangzi. So while legalism was still in full force, Taoist ideas were still evolving, granted with less vigor.

I would suspect the “take it easy” part took root due to the strictness of legalism. Indeed, there’s verses in the TTC that I see as a criticism to legalism, for example, 74.