Dual wield's Twin Blade and Blunt skill will give 5% if using daggers. You can also take the weapon enhancements on each dagger that give bonus crit for more than the bow. The playstyle may even work better with all those assassination skills (but I wouldn't count on it). There's also the Thief mundus stone which gives increased critical strike chance and the Shadow mundus stone which gives increased critical strike damage, so don't forget those.

There's some oddity with the stats sheet updating, I've noticed. For some things it simply won't update at all. This is pure theory crafting, assuming that the broken items will be fixed with time (or that the bug that prevents them from showing up is fixed if the items are working).

That said, the Shadow stone doesn't improve critical rating, but rather critical damage. Which, depending on the level of the increase that it gives, may be better than the Thief with a high enough critical value from gear.

It's possible that weapons with the precise trait (or equivalents for bows and staves) have critical damage increased, like the Shadow stone, rather than a critical rating increase (hence why it doesn't show up in the character pane) but reading the description of the trait definitely gives me the impression that they are supposed to affect critical rating rather than critical damage.

Edit: I tested precise weapons on my own. They do work and show up on the stats panel.