Hey!

 

As you yourself allude to, most of the information we possess about the Hašīšiyya is derived from polemic sources. Because I don't have my regular sources at my disposal right now, I'm going to take a look at their entry in The Encylopaedia of Islām.

Bernard Lewis mentions that while all European names for the order are derived from the Arabic term for cannabis (ḥašīš), usage of the substance is not attributed to the sect in the medieval contemporary sources of either Crusader, Sunni or Isma‘ili origin; although it appears once in a polemic Fatimid tractate. Lewis concludes that the term is of Syrian origin and was restricted to that area.

 

tl;dr: no, there is no indication that they smoked (excessive amounts of) hashish

 

Sources

• Lewis, B. "Hashshashiyya" in The Encyclopaedia of Islam, New Edition (Leiden, Brill: 1971)
  Recommended reading
  As I pointed out, I can't access it now, but also consider reading Heinz Halm's excellent book on Shi'ism, I'm pretty sure he refers to the sect in there as well.