how do i set a mados sccall step for an varargs multi-arg endpoint argument?

Since you are using a struct the ManagedBuffer inside the struc are nested encoded. Which means you need to add the length of the ManagedBuffer before it.

Luckily there is a shortcut for that by using the nested: prefix.

So your arguments would look like this:

"arguments": [
    ["str:NFT 1"],
    ["str:www.mycoolnft.com/nft1.jpg"],
    [
        ["nested:str:test1", "nested:str:test2"]
    ]
]