Generic crud with phpoo and Pdo (no framework)

Question

I want to make a kind of Generic Crud and I implemented the function of Insert, I know that the way I did is not a better way to do it and I don’t want to use any kind of frameworks and so I would like help, tips.

What is causing me more confusion is how I will do a correct and efficient processing of the data before they are inserted/edited, I did a cleaning function but I know it is more like a "gambiarra"because I like to use php’s Filters and I’m not finding a way to use them very well with object orientation and in a generic way.

I saw that it has some topics similar to mine, but I could not find what I’m looking for.

Class students:

<?php
require_once 'crud.php';
require_once 'usuarios.php';
require_once 'conexao.php';

class Alunos extends Usuarios {

    private $matricula;
    private $curso;
    private $turma;
    private $dados_cadastrais;
    private $con;

    //Métodos
    public function cadastrarAluno($dados) {
        $verifica = $this->con->conectar()->prepare("SELECT * FROM usuarios WHERE email = :email");
        $verifica->bindValue(":email", $dados['email']);
        $verifica->execute();

        if ($verifica->rowCount() > 0) {
           return false;
        } else {
            $this->dados_cadastrais = $this->limpeza($dados);
            $sql = "INSERT INTO usuarios (nome, nomepai, nomemae, datanasc, naturalidade, nacionalidade, estadocivil, sexo, endereco, complemento, bairro, cidade, estado, cep, telefone, celular, celular2, email, cpf, rg, orgaoe, estadoe, datae, formacao, cursof, senha, nivel) values (:nome, :nomepai, :nomemae, :datanasc, :naturalidade, :nacionalidade, :estadocivil, :sexo, :endereco, :complemento, :bairro, :cidade, :estado, :cep, :telefone, :celular, :celular2, :email, :cpf, :rg, :orgaoe, :estadoe, :datae, :formacao, :cursof, :senha, :nivel)";

            $cadastro = new Crud();
            if($cadastro->cadastrar($sql, $this->dados_cadastrais)){
                 return true;
            }else {
                return false;
            }
        }
    }

    function limpeza($dados){
        foreach ($dados as $key => $valor) {
            if(($key == "cpf") || ($key == "rg")){
                $_POST[$key] = filter_var($this->limpaDoc($valor), FILTER_SANITIZE_STRING);
            }else if(($key == "data") || ($key == "datae") || ($key == "datanasc")){
                 $_POST[$key] = filter_var($this->formataDataEua($valor), FILTER_SANITIZE_STRING);
            }else if(($key == "senha")){
                 $_POST[$key] = filter_var(md5($valor), FILTER_SANITIZE_STRING);
            }else{
                $_POST[$key] = filter_var($valor, FILTER_SANITIZE_STRING);
            }    
        }

        return $dados;
    }

    function formataDataEua($data){
        $data = date("Y-m-d", strtotime(str_replace('/', '-', $data)));
        return $data;
    }

    function limpaDoc($valor) {
        $valor = preg_replace('#[^0-9]#', '', $valor);
        return $valor;
    }

    //Métodos especiais
    function __construct() {
        $this->nivel = 0;
        $this->con = new Conexao();
    }

Crud class

<?php
require_once 'conexao.php';
class Crud {

    private $con;
    private $insert;
    private $update;
    private $delete;

    //Métodos especiais
    function __construct() {
        $this->con = new Conexao();
    }

    function insert($sql, $dados) {
        $this->insert = $this->con->conectar()->prepare($sql);
        foreach ($dados as $key => $valor) {
            if(is_numeric($valor)){
                $this->insert->bindValue(":$key", $valor, PDO::PARAM_INT);
            }else{
                $this->insert->bindValue(":$key", $valor, PDO::PARAM_STR);
            }
        }
        if ($this->insert->execute()) {
            return true;
        } else {
            return false;
        }
    }

}