Dear Bunpot Assawarungsee
Thanks for your quick reply!
If you start subscribing to Microsoft 365 Business (Standard), you can surely use customized domain. However, you can't use the same email address for a commercial Microsoft 365 email account and a consumer Microsoft 365 or Outlook.com account. So, your current outlook.com account and your business account would be kind of separated. You may try to set up a forwarding/redirecting rule for your outlook.com account so that the mails sent to it would be visible to your business account.
Generally, when you sign up Microsoft 365 business service, it creates a domain for you, like contoso.onmicrosoft.com. The user ID that you create when you sign up includes the domain, like ******@contoso.onmicrosoft.com.
If you want to have your email look like ******@contoso.com, you can buy a domain or just follow the steps in Add your users and domain to Microsoft 365 if you own it already. And you can keep using the initial onmicrosoft.com domain even after you add your domain. It still works for email and other services, so it's your choice.
You can also replace your onmicrosoft.com domain after sign-up. While you can’t rename or remove an existing onmicrosoft.com domain, you can create a new one and make it your default onmicrosoft.com domain (your "fallback domain"). For example, if the initial domain you chose was fourthcoffee.onmicrosoft.com, if needed you can create a new onmicrosoft.com domain called fourthcoffeecorp.onmicrosoft.com and set it to be your fallback domain.
For more information, please kindly refer to Add a domain to Microsoft 365 - Microsoft 365 admin | Microsoft Learn and Domains Frequently Asked Questions | Microsoft Learn.
Your patience and cooperation will be greatly appreciated. I hope all the best!