Speaking as a new tank myself (I have been playing other games so I'm only level 16 Glad and have tanked Sastasha only once thus far), I think you'll need at least 2x Flash at the start and likely will need to throw it in a couple of times. When I tanked Sastasha I would often have mobs pulled off of me before I could even get off a combo on more than the first target; coming from other games where it's easier to manage threat this was a shock to me To be fair though I didn't use marks, so that might have been why, although nobody died so that's a bonus.

I did find it a lot harder to regain aggro than I'm used to from other games; I would end up spamming shield lob as it is high enmity and the mob would still pretty much ignore me, but I could not run to it sometimes for fear of losing aggro on the other mobs.

I think moving forward my opening at low levels would be to use Fight or Flight, shield lob, flash x2 once mobs close in and then do a combo on one, and then do the Fast Blade on one, Savage Blade on another, perhaps with another Flash after a couple of combos (once I know I have aggro I tend to use Fast Blade -> Riot Blade to recover the MP that I use for Flash)