Wither heads farms trick in Minecraft Java?

Hello I'm playing MC on 1.16.4 and have already farmed 40 wither skeletons but unfortunately only got one head. Also use Looting 3 on my sword, does anyone have a trick for farming?

Witherskeletons drop 0 to 2 bones, 0 to 1 coal, and sometimes a stone sword. If a witherskeleton is killed by a player, there's a 2.5% chance that a witherskeleton will drop. The skulls are required to summon the wither. If the sword is equipped with the plunder enchantment, it will drop with a higher probability: 1% more per enchantment level, which results in a maximum of 5.5% at level III. If the witherskeleton dies due to the explosion of a charged creeper, there's a 100% probability that the skull will drop.