I've looked previously, but it requested SMTP address + username/password. I wouldn't have those because that custom domain email address will be forwarded (receiving only - not send). Am I missing something here?
As long as your spf record allows Google to send mail, you can.
You need to setup an "App password" on your Google account. Then, when you create an alias, you enter smtp.gmail.com and your app password credential as the smtp server.
You need to setup an "App password" on your Google account. Then, when you create an alias, you enter smtp.gmail.com and your app password credential as the smtp server.
Here: https://support.google.com/domains/answer/9437157?hl=en