You don’t have to do a long swipe, it can be quite short, about the length of the dock. The trick is not to ‘flick’ in one sudden motion but to keep your finger on the screen. It really doesn’t have to be on very long. As soon as you see the app come in from the left you can release and enter multitasking.
Another option is to double click the home button.
Another option is to swipe up with 4 fingers.
I hope one of them suits you as I’m not aware of a way to revert to iOS 11 gestures.