This is interesting. I screen a lot of ML researchers and engineers. The process typically starts with a 30-90 minute phone screen (it ends faster if I don't like you). I start by discussing the company and the position, in as much depth as the candidate wants. Then we discuss one project of the candidate's in depth, both for some technical expertise but also to suss out what the candidate, as opposed to a team, actu…
It sounds like the main problem is your focus on hiring senior people. Maybe you should go with Reid Hoffman's advice and try a junior person making a mid-career change. There will be more risk, but you stand to gain top-notch workers who the rest of the market ignore due to conservatism and credentialism.
Oh, we do hire junior people as well; it's just that you need at least a core of senior folks.