A three-dimensional memory structure includes an alternating stack of insulating layers and electrically conductive layers located over a substrate, an array of memory stack structures extending through the alternating stack, an array of drain select level assemblies overlying the alternating stack and having a same periodicity as the array of memory stack structures, drain select gate electrodes laterally surrounding respective rows of the drain select level assemblies, and a drain select level isolation strip located between a neighboring pair of drain select gate electrodes and including a pair of lengthwise sidewalls. Each of the pair of lengthwise sidewalls includes a laterally alternating sequence of planar sidewall portions and convex sidewall portions.