If you want a flail-like weapon, the closest you’ll likely ever get to unlocking one would be Ko’gun, Hammer of the Firelord which can drop from the Cache of the Fire Lord after defeating Ragnaros at the end of the Firelands raid.
Blizzard seems dead set on never adding a new flail appearance ever again.
. People that already completed the challenges should get all the rewards and the achievement. I’m serious. Re-opening the mage tower was supposed to appease players that failed or weren’t around to do it during Legion.