Thanks for the comments.
The base encumbrance on the Shadow Priestess is an oversight. Looks like a I plain forgot to add the #enc command and missed it when I looked at the mod in-game, so it defaults to 10. Should be 1. You can fix that by just adding #enc 1 to the Shadow Pirestess unit in the mod file, it'll retroactively fix your games that use the mod.
As far as making the Priestess and the Grand Thaumaturg competitive with each other, what about upping the base D1 on the Priestess to D2 and a cost of around 180? That's a D2 mage with possible more magic paths, low base encumbrance, fast mapmove to shuffle undead armies around, massive undead leadership and immortal to boot, sounds fairly good to me that way, but I worry about making the GT too unattractive. Though the GT is the only one who can get Air and H3, and has the highest potential for Death and outright highest Astral.
The unit costs are something I just basically drew out of a hat, which is why I need feedback such as yours. I suck at the cost analysis side.
The idea with the Renata and Renatus was to give the nation easier access to Water and Nature magic, but not too high levels. They can get access to both Voice of Apsu and Haruspex in addition to Dark Knowledge and Arcane Probing, so chances to diversify magic later in the game is increased. The Renatus also gives a chance to get D2 mages without requiring a Grand Thaumaturg. The Thaumaturg is indeed an excellent mage and I'm having hard time pinpointing where a sweet spot of balance so that all of the mages would be usable, because there is so much overlap between the Thaumaturg and the Renatus.
Would these ideas work?
I would also like to get feedback from others who have tried this mod.