My general rule of thumb is that a tree below ground is a near mirror image of the tree above ground. Just without the leaves. So if you have a broad crown tree, the roots will spread. If you have a conical tree like most thujas, you will have a central tap root. It is very hard to transplant those tap-rooted trees after they have had opportunity to grow in the ground.
If you are able to move yours successfully, I have found nowhere that says they are a specific danger to septic areas.
I want to know how this goes for you, Sunny.