1) I hope you understand that "belated" means "late." You only use "belated" if you didn't send the greeting on time. For example, suppose it is November 19th. If someone has a birthday on November 22nd I'd say "Happy birthday!" But if their birthday had happened yesterday, I'd say "Please accept my belated birthday greeting." 2) "Please accept my belated birthday greetings" is fine. It is fine without the "please" but it never hurts to add "please." 3) With all such message, "it's the thought that counts." What's important is the simple fact that you've sent them a birthday message. How it is phrased hardly matters. Don't worry about the exact words you use.

That’s perfect English. It sounds a bit formal, but in sort of a friendly, not quite serious way. Of course, if you’ve forgotten the birthday of someone close, it really doesn’t matter *what* you say….

Thank you!

Yes that sounds perfect. I would like to add that It also sounds formal.