I wasn’t able to find any way to specify complex validation rules like “If A > 10, then B should have one of {C,D,E}”. Specific example is here.
Unfortunately, this isn’t possible using the Terraform Plugin SDK at this time. We’re currently working to address this shortcoming, and hope to have more information on that front soon.
Would you think that having ValidateDiagFunc on the level of Resource should ultimately solve the problem? It’s the least invasive change that I can think of.