You can specify recursive types in the typing language by using type aliases and forward reference strings,

Garthoks = Union[Garthok, Iterable['Garthoks']]

Mypy supports recursive types by default since v0.990, and Pyright/Pylance since v2020.9.4.

Some types of forward references are handled by PEP 563. You can use them starting from Python 3.7 by doing from __future__ import annotations – Konstantin

As of Python 3.12, __future__.annotations/stringifying is not necessary if the type is defined using a type statement:

type Garthoks = Garthok | Iterable[Garthoks]

Assuming that you decide to retain _check_parameters(), here's an illustration of some alternative techniques for organizing a complex boolean check. (1) The isinstance() function will take a tuple of types, so you can do some consolidation. (2) If you need to check for None and check for types, you can do it all in one shot. (3) Some of your checks were repetitive; help the reader by factoring things out. (4) Organize the checks like a pretty-printed data structure because it gets the parens/brackets working for you to convey logical structure. (5) Sometimes simple and fairly banal code comments can function as visual/organizational sign posts to guide the reader. (6) I prefer the lines of code to lead with the substance rather than the boolean operator -- which is mostly a stylistic preference, but I do think it combines better in these kinds of complex checks. For example, I felt no readability-driven urge to add a preliminary True and to the expression. Also, most people use editors with syntax highlighting, so the operators pop out visually and there's no need to waste the prime real estate (the start of each line) on the operator.

TNone = type(None)

check_ctype_seq = lambda ctypes, cls: (
    isinstance(ctypes, cls) and
    all(
        isclass(ct) and
        issubclass(ct, GenericContainer)
        for ct in ctypes
    )
)

return (
    # Length and depth.
    isinstance(length, (int, Callable, TNone)) and
    isinstance(depth, (int, Callable, TNone)) and
    # Matching type.
    (
        isinstance(matching_type, (TNone, type)) or
        is_n_list(matching_type, None, type) or
        is_n_tuple(matching_type, None, type)
    ) and
    # Container_type.
    (
        container_type is None or
        (
            isclass(container_type)
            and issubclass(container_type, GenericContainer)
        ) or
        check_ctype_seq(container_type, list) or
        check_ctype_seq(container_type, tuple)
    )
)

What you need is a TypeForm from https://peps.python.org/pep-0747/. Which has now been released with typing_extensions 4.13.

If want to strictly insist on a list, the code at the bottom will.

Alternatively go for if any sequence is acceptable:

type ValidatableAnnotation = (
    str | int | float | bool | Sequence["ValidatableAnnotation"]
)

Second note, that you have to differentiate between a TypeForm[ValidatableAnnotation] which can just be a string representing a type and a ValidatableAnnotation which will be a valid object.

Code sample in pyright playground

from typing_extensions import TypeForm, Type, Any, Sequence, TypeVar

# Define valid types here or in a variable/typea alias
type ValidatableAnnotation[typeT: str | int | float | bool] = (
    typeT | list["ValidatableAnnotation[typeT]"]
)

def validate(value: Any, type_: TypeForm[ValidatableAnnotation]):
    ...

validate([1], list[int]) 

# Not a Type Expression, should be rejected
x_form: TypeForm[ValidatableAnnotation] = [int, float]
# Okay as a value
x_value: ValidatableAnnotation = [int, float]

y: TypeForm[ValidatableAnnotation] = list[int]

z: TypeForm[ValidatableAnnotation] = list[float]
u: TypeForm[ValidatableAnnotation] = list[list[int]]

As of mypy 0.990, mypy finally supports recursive type annotations, using the natural syntax:

from typing import Union, Dict, List

JSONVal = Union[None, bool, str, float, int, List['JSONVal'], Dict[str, 'JSONVal']]

d: JSONVal = {'a': ['b']}

mypy output:

Success: no issues found in 1 source file

Before 0.990, this would produce an error reporting a lack of recursive type support:

$ mypy asdf.py
asdf.py:3: error: Recursive types not fully supported yet, nested types replaced with "Any"

On such versions, Dict[str, Any] would be the way to go.

You can also use mutually recursive type aliases now, so you can do things like

from typing import Union, Dict, List

JSONVal = Union[None, bool, str, float, int, 'JSONArray', 'JSONObject']
JSONArray = List[JSONVal]
JSONObject = Dict[str, JSONVal]

d: JSONObject = {'a': ['b']}

You can use Forward References as defined in PEP 484

A situation where this occurs commonly is the definition of a container class, where the class being defined occurs in the signature of some of the methods. For example, the following code (the start of a simple binary tree implementation) does not work:

class Tree:
    def __init__(self, left: Tree, right: Tree):
        self.left = left
        self.right = right

To address this, we write:

class Tree:
    def __init__(self, left: 'Tree', right: 'Tree'):
        self.left = left
        self.right = right

It is allowable to use string literals as part of a type hint, for example:

class Tree:
    ...
    def leaves(self) -> List['Tree']: