I tried Kesarai.5084’s method of starting at the third tower and working backwards, and was finally able to complete this quest. Prior to that, I’d tried twice only to have the final cutscene fail to kick in after completing the “Clear the Beach” objective.
I’m not sure whether it was ensuring that the spawned mobs were the last to die, or if the fact that I finally managed to kill the Risen Thrall that spawns inside the first tower (and usually gets trapped in the rubble, often hard to see his name unless you’re beach side and catch a glimpse) therefore ACTUALLY clearing the beach.