Ditto
I'm not sure Prod is very useful, even with Prod-0 you can crank 200 res a turn per castle, largely enough for legions of Principes
I'd rather go for Growth to alleviate aging, and maybe a Nature-5+ pretender for GoH (plus the mages will regenerates.. hum yes it's nearly useless)- even if getting the gems won't be that easy.
Also I'm not a fan of a Bless strategy with the capitol-only and rather weak Vestals, rather an all-out Astral magic strategy using Communions to unleash the real nasty spells such as Arcane Domination and Master Enslave.
With that in mind an Astral-9 Nature-5 Oracle would be a good choice, and you'll even get a nice bless eventually