That's a clever solution.
Six is a little bit Young to be able to have an eye for the intricacies of a form letter but why not make something that looks official (think of contract paperwork or something you would get from a utility company or the government... With a logo, make it look official)
And in child friendly but still adults sounding tone (if you're having a hard time coming up with the right wording, try chat gtp. If you don't subscribe to it you can use it for free, all you have to do is ask it for exactly what you want and it can adjust the tone of the letter for you, it's quite a clever writing tool) just explain that due to health and safety regulations, it's not safe for children to have real jetpacks or even a visibility cloaks. That these things would seem fun, but it's the job of adults to make sure that children remain safe.
That sometimes it's not fun as an adult to say "no" but to trust that the adults that care about you have your best interest at heart...and that includes your mummy and daddy.
Finish off with a statement something like due to international safety regulations, Santa is only permitted to deliver the type of toys that can be found in shops, and to help you, your mommy can show you some store flyers/catalogs (if you want to do that... Might help to direct his wish list into a category / price range that you can afford)
Maybe sprinkle a little tiny bit of glitter on the envelope and the letter so it looks a little more magical?
While I agree that six year old is too young to break the news of how Santa really works, 6-year-old is not too young to reinforce the idea that your parents (and other adults who can care for you) have the responsibility of making sure that you grow up safe and sometimes that means imposing rules that they don't like. Cuz that's life, and that lesson always needs to be reinforced.
Now on that note, I'm going to do something that I don't really want to do but I'm doing to protect my parents, I'm going to go get my flu shot.