Onshore Correspondent Banks for Offshore Accounts – How Much Diligence is Enough?

Still trying to wrap my head around the level of due diligence required when vetting onshore correspondent banks for offshore accounts. Beyond the standard KYC/AML checks, what specific red flags or deep dives are you guys doing to assess their stability and willingness to handle these types of transactions without suddenly freezing funds or closing accounts? Feels like there's always a hidden layer.