Hi @kderck 
It seems as though your description of CustomConditionModel.Value aligns with dynamic types. There has been some discussion around the usage of dynamic types, for instance Terraform-plugin-framework map of different value types and Support Dynamic Type/Attributes/Parameters on GitHub.
Dynamic type support is being considered, but has not yet been implemented in the framework. Refer to https://discuss.hashicorp.com/t/terraform-plugin-framework-map-of-different-value-types/56421/7:
Your only option in this case might be to use a dynamic pseudo type from terraform-plugin-go along with muxing. There is currently an open issue on terraform-plugin-framework for Support Dynamic Type/Attributes that discusses this. But until such time that this is added to the framework it may be necessary to use terraform-plugin-go along with muxing.