Terraform 1.9 introduced some time ago the capability to reference in an input variable validation condition other variables, not only the one you're validating.

What does not work is having two variables which validate each other, e.g.

variable "nat_min_ports" {
  description = "Minimal amount of ports to allocate for 'min_ports_per_vm'"
  default     = 32
  type        = number
  validation {
    condition = (
      var.nat_min_ports >= 32 &&
      var.nat_min_ports <= 32768 &&
      var.nat_min_ports < var.nat_max_ports
    )
    error_message = "Must be between 32 and 32768 and less than 'nat_max_ports'"
  }
}

variable "nat_max_ports" {
  description = "Maximal amount of ports to allocate for 'max_ports_per_vm'"
  default     = 16384
  type        = number
  validation {
    condition = (
      var.nat_max_ports >= 64 &&
      var.nat_max_ports <= 65536 &&
      var.nat_max_ports > var.nat_min_ports
    )
    error_message = "Must be between 64 and 65536 and above 'nat_min_ports'"
  }
}

That let directly to the following rather opaque error message: Received an error Error: Cycle: module.gcp_project_network.var.nat_max_ports (validation), module.gcp_project_network.var.nat_min_ports (validation)

Removed the sort of duplicate check var.nat_max_ports > var.nat_min_ports on nat_max_ports to break the cycle.