Is this for PvP you're talking? If it's PvE I'm surprised that Dire would be considered at all better than Carrion or Rabid or a mix of those two. Thieves are best at, you know, doing lots of damage.
You need to get your sustain from mobility and trickery for the most part unless this is meant to be some sort of condi support build where damage isn't your biggest priority, but it doesn't sound like that. Consider at least shadow refuge and possibly, shadowstep and blinding powder- all three if you need them. Otherwise caltrops for more bleeds. If you're in trouble, use stealth to reposition rather than for the attack. Sneak attack from shadow refuge also does lifesteal.
Also, it sounds like you're focusing on getting ALL your bleed from Sneak Attack with pistol? You can and should also spend initiative on rapid bleeds by making d/d your other set. Death Blossom with high condi will make groups kinda melt away in many PvE settings.
If you really want to go all out with this, I suggest Sigils of Earth and Geomancy and Superior Runes of the Krait along with Basilisk Venom.
Bear in mind, even with all this trickery, hitting a 25 stack by yourself as a thief is just not practical, that's not what you're aiming for.
Also bear in mind that in PvE, direct damage is rather likely a better option for thief almost all of the time. Conditions ignore armor which is awesome, but thieves can pump out HUGE numbers in spikes a variety of ways. I have a hybrid condition build myself as one of mine, and it's a lot of fun, but I'm just saying, if you're not happy with the damage you're getting with it at present...