You're not thinking with (time) portals enough, with short skips backwards you could track them back through time to the point of planning, you could catalogue the entirety of al-qaeda from that point with enough patience. Why stop the plane attacks when you could dismantle and disgrace the entire organisation and salt the earth. Kill bin-laden and bring back some deep-fakes tech to make a convincing video that makes it look like he died in an autoerotic asphyxiation accident looking at some furry porn of Mohammed and an anthropomorphised goat-dude. Go back to all his lieutenants' last medical appointment and give them all HIV and release a video of gay orgies with their faces pasted on shortly after bin Laden's dead and let the bizarrely specific cases of AIDs cement that narrative and the org will tear itself apart. Send the rest of the info you gathered to the CIA before you move on so they can tidy up and any loose ends.

And that's only if you don't feel like traveling a lot further back and just giving a 9 y/o girl a knife and some self-defence pointers before her wedding night and really nipping the whole shebang in the bud.

The world is your vengeful oyster, think big.