I won't ask for "What should I buy next", but I want to gather some ideas and experiences.
I am looking for my next lens and let me give you the facts:
Sigma 30mm 1.4
Tamron 70-300mm VC
Canon 50mm 1.8 (not used for years)
Speedlite 430EX II
A few comments:
Low light zoom: Depending on what "low light" means, f2.8 may not be fast enough, not sure if OS helps in every situation, and my preference is a fast prime or Flash.
With a flash, you may as well stick with one of the zooms. I am generally very happy with IQ with just bouncing the flash off the ceiling, without getting into any more involved lighting setups.
For a fast prime...if there is freedom to move around, say at family events, I usually can get buy with a prime. I have the Sigma 30mm, and also the Canon 85mm/1.8. I bought the Canon primarily to increase distance to pets and kids playing, where flash or getting too close with the 30mm might disrupt the activity.
I generally like working with camera and smaller lenses, in order to be less intrusive, have less to carry around, and I am willing to trade-off having to get a less-than ideal framing of a scene and crop later.
If your Sigma 30 is problematic (mine is excellent), consider replacing it. Canon 35/2, 35/1.4, etc.
(BTW...noted your username...just now reading the books for the first time)