But the harmony tree has not received new content to make it more appealing.
No it has not, and a mod is not likely to add new content, merely change what's already there...
That being said, certain techs (such as the higher end metal/crystal extraction increases) are pretty uneconomical for everyone...boosting or improving those would make civil labs more useful for all three factions...for Advent in particular, having a better resource focus and a better Allure of the Unity would make that 7th and 8th lab at least have some purpose...
Buffing the culture techs in general (such as an increase to max allegiance) and/or to the deliverance engine would also help the Advent civilian tree...
It would require a lot of changes and modding, but I do believe it is possible to make the harmony tree at least useful without the addition of new content...the Advent civil tree probably will never compare to the TEC's or Vasari's without new content, especially if improvements to the Advent harmony tree are "blanket" improvements that help everyone...but being useful is a pretty good improvement from what we have, and honestly I'm okay with Advent having a weaker civil tree if they make up for it elsewhere...