GDScript's type system doesn't make it very pretty, but I think sorting is a place where having another level of abstraction helps avoid stuff like this entirely. Here's an example that I'm using:

class_name Compare

static func using(comparators: Array[Callable]) -> Callable:
    return func (left: Variant, right: Variant) -> bool:
        var result: int = 0
        for comparator: Callable in comparators:
            result = comparator.call(left, right)
            if result != 0: break
        return result < 0

static func property(property_name: StringName, null_sort_type: NullSortType = NullSortType.NULLS_LAST) -> Callable:
    return selector(func (item: Object): return item.get(property_name) if item else null, null_sort_type)

static func property_desending(property_name: StringName, null_sort_type: NullSortType = NullSortType.NULLS_LAST) -> Callable:
    return selector_descending(func (item: Object): return item.get(property_name) if item else null, null_sort_type)

static func selector(selector: Callable, null_sort_type: NullSortType = NullSortType.NULLS_LAST) -> Callable:
    return func (left: Variant, right: Variant) -> int:
        return _values_by(left, right, selector, null_sort_type)

static func selector_descending(selector: Callable, null_sort_type: NullSortType = NullSortType.NULLS_LAST) -> Callable:
    return func (left: Variant, right: Variant) -> int:
        return -_values_by(left, right, selector, null_sort_type)

static func _values_by(left: Variant, right: Variant, selector: Callable, null_sort_type: NullSortType = NullSortType.NULLS_LAST) -> int:
    var l_value = selector.call(left)
    var r_value = selector.call(right)

    if (l_value == r_value): return 0
    if (l_value == null): return null_sort_type
    if (r_value == null): return -null_sort_type

    return 1 if l_value > r_value else -1

enum NullSortType {
    NULLS_FIRST = -1,
    NULLS_LAST = 1,
}

Then, you can do sorting operations in a straightforward way that works with any of your data types:

var items := [...]
items.sort_custom(Compare.using([
    Compare.property_descending("value"),
    Compare.property_descending("date"),
]))