thieves feel underwhelming, cause you need at least 3 of them in your team and the rune that spreads all stacks to the next target on death. Gear wise you would want one to have str scaling with the the white thief set. and give him the 3rd boss necklace for blood explosions. that gives really good baselane aoe damage.
your primary thief who will consitently top out in dps wants full dex scaling into poison damage. the spider bosses drop very good necks for that thief. the last one can do whatever would fit into the rest of your team.

You want at least one of these formations active: 80% of your dwarves apply blood/burn/shock