So fire them. It's bizarre that companies appear to think that you should do cargo cult interviewing, sometimes multiple rounds with months of waiting around, then hire people, fail to lead or develop them, then just dump them en masse.

Hire them and if you can't get the performance you need, then you due them. Alternatively, hire them as contract-to-hire if your head of HR has conditioned you to be terrified of doing what you're allowed to do in the US.