Given a trader llama and a wandering trader (I use them with NoAI to keep them from wandering off, which I don't think is relevant), I get the trader's UUID as an array with

/data get entity 1234.... UUID

(using completion to get the trader's target UUID). This gives me a four int array, which I copy from the log. I then paste it into the following command (again using completion to fill in the trader llama's UUID for the entity target):

/data modify entity 5678... leash.UUID set value [I; 1234... ]

... and the trader llama is destroyed, it goes away. This works fine with other animals I've tried, just tender llamas.

I've seen MC-258553, but things have changed since then. That problem was resolved by setting PersistenceRequired on the llama. But now that no longer works. I explicitly set it on the trader llama when I summoned it, and then checked before the executing the above commands that it was set (to 1b).