The tone problem, in one paragraph
A fortieth invitation usually wants to be funny and ends up slightly embarrassing. The trick is to save the jokes for the room rather than the page. Write what the evening will feel like, name the venue and the time without a riddle, and let the humour arrive in the speeches, the playlist or the photographs. Nobody keeps a poster from a group chat; people keep a page they can open again on the way there.
Because birthdays are a real format here rather than a workaround, the page can carry a proper design and the sections guests expect, with the reply wired underneath. Dozens of designs are in the library, the wording is yours to rewrite, and the count of who is coming is what makes the whole thing useful to you.