hyperparameter tuning with validation set

For what I know, and correct me if I am wrong, the use of cross-validation for hyperparameter tuning is not advisable when I have a huge dataset. So, in this case it is better to split the data in training, validation and test set; and then perform the hyperparameter tuning with the validation set.

In the case that I am programming I would like to use scikit, the yeast dataset available at: http://archive.ics.uci.edu/ml/datasets/yeast; and for example to tune the number of epochs.

First, I have separated my training, validation and test set by using the train_test_split twice according to one answer that I saw here. The loss plot that I got is the following for 1500 max iterations:

Then I wanted to use my validation set with a list of different values for the hypeparameter of max iterations. The graph I obtained is the following (with some warning messages of non-convergence for max_iter values less than 1500):

So, I have the first question here. It seems that for a value of max_iter of 3000 the accuracy is 64% approximately, so I should choose that value for the max_iter hyperparameter; is that correct? I can see from the graph that also the red line of 3000 has a less value of loss than the other compared options.

My program so far is the following:

import numpy as np
import pandas as pd
from sklearn import model_selection, linear_model
from sklearn.model_selection import train_test_split
from sklearn.metrics import confusion_matrix
from sklearn.neural_network import MLPClassifier
import matplotlib.pyplot as plt
from sklearn.model_selection import GridSearchCV

def readFile(file):
    head=["seq_n","mcg","gvh","alm","mit","erl","pox","vac","nuc","site"]
    f=pd.read_csv(file,delimiter=r"s+")
    f.columns=head
    return f

def NeuralClass(X,y):
    X_train,X_test,y_train,y_test=model_selection.train_test_split(X,y,test_size=0.2)
    print (len(X)," ",len(X_train))
    X_tr,X_val,y_tr,y_val=model_selection.train_test_split(X_train,y_train,test_size=0.2)
    mlp=MLPClassifier(activation="relu",max_iter=1500)
    mlp.fit(X_train,y_train)
    print (mlp.score(X_train,y_train))
    plt.plot(mlp.loss_curve_)

    max_iter_c=[500,1000,2000,3000]
    for item in max_iter_c:
        mlp=MLPClassifier(activation="relu",max_iter=item)
        mlp.fit(X_val,y_val)
        print (mlp.score(X_val,y_val))
        plt.plot(mlp.loss_curve_)
    plt.legend(max_iter_c)

def main():
    f=readFile("yeast.data")
    list=["seq_n","site"]
    X=f.drop(list,1)
    y=f["site"]
    NeuralClass(X,y)

Second question, is my approach valid? I have seen a lot of information over the web and all point to cross validation for hyperparameter tuning, but I want to perform it with a validation set.

Any help?

PD. I have tried early stopping and the results are poor compared to the ones obtained with the method I programmed.

Thanks