Lastly, if you want to go really big, build a decoration with the "Rouse Matrix System". This is an expandable, light weight framework to hold balloons together.
Rouse Matrix comes in lots of shapes including hearts, stars, and rounds as well as banner pieces which can either be joined together to create backdrops or cut to your choice of specific shapes.
Find out more about the Rouse Matrix System at their website and in the short explanation video below.
If you decide to try the Rouse Matrix System, I'd recommend to contact them first (there's an email address on their website). Last posts on their social media channels were in 2020 — so I am not sure if they are still in business.
Good luck with creating some stunning balloon decorations for your birthday party!