We have a variety of ways to keep triggering Kardur: the most common is by making token copies of Kardur with cards like Jaxis, the Troublemaker and Orthion, Hero of Lavabrink which trigger his mega goad before dying, but we also have bounce with Skull Collector and reanimate/blink with Rescue from the Underworld
But as is, if you have the mana, Legacy Weapon is the answer
And why are collectors so determined to acquire them
Were also not going to count serialized cards, since they are intentionally printed at a limited number and can skew the top end of the game to just a bunch of serialized cards, and thats not fun
The rules for layers are notoriously tricky, but in almost all cases you apply the copy effect BEFORE other stat changes, meaning lingering buffs from triggers will boost its new power too