So here is my interpretation of your SObjectBuilder or TestFactory :) - it ain't static btw !! I did not compile this...just jotted it down and put it together how it made sense to me. Probably makes sense to put a Util class back in for the describes and as you can see below using it should be really simple. I think it depends a bit if you like…

AccountBuilder.cls

@isTest public with sharing class AccountBuilder {
  private Account testAccount;
  
  public AccountBuilder() {
  	this.testAccount = (Account)new SObjectBuilder('Account').build();
  }
  
  public AccountBuilder withRecordType(String recordTypeLabel) {
    this.testAccount.RecordTypeId =
      Schema.SObjectType.Account
        .getRecordTypeInfosByName()
        .get(recordTypeLabel)
        .getRecordTypeId();
    return this;
  }

  public AccountBuilder withName(String Name) {
    this.testAccount.Name = Name;
    return this;
  }

  public AccountBuilder insert() {
  	insert this.testAccount;
  	return this;
  }

  public Account build() {
    return this.testAccount;
  }

  @IsTest
  private static void AccountBuilder_insertAccount_Success() {
    Test.startTest();
      Account testAccount = new AccountBuilder()
      							.insert()
                        		.build();
    Test.stopTest();

    System.assertNotEquals(null,
                           testAccount.Id);
  }
}

ContactBuilder.cls

@isTest public with sharing class ContactBuilder {
  private Contact testContact;
  
  public ContactBuilder() {
  	this.testContact = (Contact)new SObjectBuilder('Contact').build();
  }
  
  public ContactBuilder withRecordType(String recordTypeLabel) {
    this.testContact.RecordTypeId =
      Schema.SObjectType.Contact
        .getRecordTypeInfosByName()
        .get(recordTypeLabel)
        .getRecordTypeId();
    return this;
  }

  public ContactBuilder withFirstName(String firstName) {
    this.testContact.FirstName = firstName;
    return this;
  }

  public ContactBuilder withLastName(String lastName) {
    this.testContact.LastName = lastName;
    return this;
  }

  public ContactBuilder withEmail(String email) {
    this.testContact.Email = email;
    return this;
  }

  public ContactBuilder withContactType(String ContactType) {
    this.testContact.Contact_Type__c = ContactType;
    return this;
  }

  public ContactBuilder insert() {
  	insert this.testContact;
  	return this;
  }

  public Contact build() {
    return this.testContact;
  }

  @IsTest
  private static void ContactBuilder_insertContact_Success() {
    Test.startTest();
      Contact testContact = new ContactBuilder()
      							.insert()
                        		.build();
    Test.stopTest();

    System.assertNotEquals(null,
                           testContact.Id);
  }
}

How to use it

// simple contact
Contact actualContact = new ContactBuilder()
			    .insert()
			    .build();

// simple contact related to simple account
Contact actualContact = new ContactBuilder()
			    .withAccount(new AccountBuilder()
				             .insert()
     					     .build())
			    .insert()
			    .build();

// set some specific values you care about in code
Contact actualContact = new ContactBuilder()
			    .withFirstName('Florian')
			    .withLastName('Stealing Some Code')
			    .withAccount(new AccountBuilder()
			                     .withName('Brain Trust')
				             .insert()
				             .build())
			    .insert()
			    .build();

SObjectBuilder.cls

@isTest public without sharing class SObjectBuilder {
	private SObject testSObject;
	
	public SObjectBuilder(String sObjectType) {
		this.testSObject = Schema.getGlobalDescribe().get(objectType).newSObject();
		this.populateRequiredFields(sObjectType);
	}
	
	public SObject build() {
		return this.testSObject;
	}
	
	private void populateRequiredFields(String sObjectType) {
        DescribeSObjectResult objectDescribe = Schema.getGlobalDescribe().get(sObjectType).getDescribe();
        // Booleans can't be required. Lookups are too complex. Neither are considered here.
        for (Schema.SObjectField soField : objectDescribe.fields.getMap().values()) {
            Schema.DescribeFieldResult fieldDescribe = soField.getDescribe();
            if (isRequired(fieldDescribe)) {
                Schema.DisplayType displayType = fieldDescribe.getType();
                if (isText(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), 'Default text');
                }
                else if (isNumber(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), 42);
                }
                else if (isDate(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), Date.today());
                }
                else if (isPhone(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), '5555555555');
                }
                else if (isEmail(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), 'defaultEmail@testdata.com');
                }
                else if (isUrl(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), 'https://mavens.com');
                }
                else if (isPicklist(displayType)) {
                    defaultSObject.put(fieldDescribe.getName(), fieldDescribe.getPicklistValues()[0].getValue());
                }
            }
        }
    }
    
    private Boolean isRequired(Schema.DescribeFieldResult describeResult) {
        return !describeResult.isNillable() && describeResult.isUpdateable();
    }

    private Boolean isText(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.String || displayType == Schema.DisplayType.TextArea;
    }

    private Boolean isNumber( Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.Currency ||
               displayType == Schema.DisplayType.Double ||
               displayType == Schema.DisplayType.Integer ||
               displayType == Schema.DisplayType.Percent;
    }

    private Boolean isDate(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.Date || displayType == Schema.DisplayType.DateTime;
    }

    private Boolean isPhone(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.Phone;
    }

    private Boolean isEmail(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.Email;
    }

    private Boolean isUrl(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.URL;
    }

    private Boolean isPicklist(Schema.DisplayType displayType) {
        return displayType == Schema.DisplayType.Picklist || displayType == Schema.DisplayType.MultiPicklist;
    }
}

insert is a reserved word, so your class won't compile sadly if you use it as your method name :)