Under Gestures and Motions in settings, there should be a setting to take a screenshot when swiping the edge of your hand across edge of screen. I have never used that so don't know if it'll serve as a suitable replacement for what you're looking for.
It would be nice if Samsung offered gestures as triggers for Routines, then one could just create a Routine to do what you're looking for (because take a screenshot is an existing action available under Routines). Otherwise, I don't think one can customize gestures very much beyond what Samsung default provides (and they took away some of it a bit ago).