Does saved Dao insight boost passive Qi? In other words is there any point to rebirthing after you've gotten all the Dao nodes?
No. Dao Insight is only a currency for awakening nodes on the Dao Tree. Unspent Insight does not boost passive Qi or anything else. Once all twelve nodes are awakened there is no mechanical reason to reincarnate again. The tree is the endgame, and the True Immortal popup at stage 89 is the finish line.
BUT
I may add a small stacking bonus per unspent Insight, or a repeatable final node in a future release.