1e Coautls were very psi oriented; maybe historical justification?
But Psi Charm as a (ps) is rather juicy.
Needs psi Mindbender 1.
2rhd+2la+6ecls buys off by level 20.
4rhd+2la+6ecls isn't quite there by level 20.
I'd say a 19 level build with exp to play with.
So 4rhd with 15 Class Levels and exp burn.
The non-evil UrPriest is thematically OK.
The coautl don't really worship any gods
and are part in the good serpent split from
ahriman (asmodeus); ancient d&d timeline.
Anyway not a lot of choices if you wanna keep up.
6 CL with only 4hd is kinda interesting (but at level 6).
One of the CMage reserve feats might do here.
Ardent 1 for a progression / Slayer X can work.