You can assign to each contact a message type (for example, "Formal", "Informal" or "Funny") and prepare messages for each type in advance. You can also create new message types in case you need them.
You can give each contact a nickname that will be inserted in the messages by replacing a keyword. This way you can create generic messages that will seem personal.
On the contacts birthday you will receive a notification that will take you directly to the available messages screen and you can quickly send one via the Messages app or via Whatsapp.
You can set the time when you want to receive the notification on the birthday and also a second alert for the day before, two days before or a week before.
There is also a notes field dedicated to each contact where you can indicate useful information you want to remember, for example gift ideas.
Add the widget to the Home Screen to keep an eye on the next Birthday and be prepared.