Which stats a dot/doom bolt conjurer should invest?

Str %35
Cunning %5
Spirit %60

How is it?

Physique 90%.
Cunning… maybe, if needed to use weapon.
Spirit enough only to use gear.

A minor issue in the game balance is that attribute balance is trash atm, and the only reason you want either spirit or cunning is just to use your weapons and armors that need it, everything else should go into physique.

Spirit increases vitality and chaos damage?
Why shouldn’t i invest into that?

Because the amount of damage boost you get from spending attribute points into spirit is very small relative to the damage boosts you’ll end up getting elsewhere. If you end up with +1000% chaos damage from your equipment and devotion bonuses, is bumping that up to +1005% with attribute points really gaining you very much?