Vega-Lite: Making overlay text look good on a diverging scale

hardcoded.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"url": "data/cars.json"},
  "transform": [
    {
      "aggregate": [{"op": "count", "as": "num_cars"}],
      "groupby": ["Origin", "Cylinders"]
    }
  ],
  "encoding": {
    "y": {"field": "Origin", "type": "ordinal"},
    "x": {"field": "Cylinders", "type": "ordinal"}
  },
  "layer": [
    {
      "mark": "rect",
      "encoding": {
        "color": {
          "field": "num_cars",
          "type": "quantitative",
          "title": "Count of Records",
          "legend": {"direction": "horizontal", "gradientLength": 120},
          "scale": {
            "scheme": "redblue"
          }
        }
      }
    },
    {
      "mark": "text",
      "encoding": {
        "text": {"field": "num_cars", "type": "quantitative"},
        "color": {
          "condition": {"test": "27.75 < datum['num_cars'] & datum['num_cars'] < 81.75", "value": "black"},
          "value": "white"
        }
      }
    }
  ],
  "config": {
    "axis": {"grid": true, "tickBand": "extent"}
  }
}

not-hardcoded.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"url": "data/cars.json"},
  "transform": [
    {
      "aggregate": [{"op": "count", "as": "num_cars"}],
      "groupby": ["Origin", "Cylinders"]
    },
    {
      "joinaggregate": [
        {
          "op": "min",
          "field": "num_cars",
          "as": "num_cars_min"
        },
        {
          "op": "max",
          "field": "num_cars",
          "as": "num_cars_max"
        }
      ]
    },
    {
      "calculate": "(datum.num_cars_min + datum.num_cars_max) * 0.25",
      "as": "lo_mid"
    },
    {
      "calculate": "(datum.num_cars_min + datum.num_cars_max) * 0.75",
      "as": "hi_mid"
    }
  ],
  "encoding": {
    "y": {"field": "Origin", "type": "ordinal"},
    "x": {"field": "Cylinders", "type": "ordinal"}
  },
  "layer": [
    {
      "mark": "rect",
      "encoding": {
        "color": {
          "field": "num_cars",
          "type": "quantitative",
          "title": "Count of Records",
          "legend": {"direction": "horizontal", "gradientLength": 120},
          "scale": {
            "scheme": "redblue"
          }
        }
      }
    },
    {
      "mark": "text",
      "encoding": {
        "text": {"field": "num_cars", "type": "quantitative"},
        "color": {
          "condition": {
            "test": "datum['lo_mid'] < datum['num_cars'] & datum['num_cars'] < datum['hi_mid']", 
            "value": "black"
          },
          "value": "white"
        }
      }
    }
  ],
  "config": {
    "axis": {"grid": true, "tickBand": "extent"}
  }
}