despawn everything spawned on wave end?

There's an EliminationWaveCompleted method in the WaveSyncroSpawnerListener class that you could use if it's a global elimination wave. Then you could add code to Despawn all of whatever item spawns from spawner X, assuming it only spawns from that particular spawner (PoolBoss method).

Or you could get the collection of items spawned from the WaveSyncroPrefabSpawner class, property: SpawnedWaveMembers and despawn them all in a loop from Poolboss.