Having to buy the manuals separately after the fact isn't the same. It's all about the unboxing experience and having something to read on the bus or walk home or when you receive a game on Christmas Eve before you get home to play it (we open gifts on the 24th not the morning of the 25th like some places)
That said if I could get manuals for all of my Switch games, I might.
Good idea sending one to Elliot btw. But you've now given him a difficult choice. Each time he builds a new custom Game Boy or receives one in the mail he has to choose between the official dev test cart and yours. And there's a pretty big geeky coolness factor to an official dev cart.
personally I would prefer a good old cardboard box though I actually made a few replacement cardboard boxes myself, might want to make a simple print at home template for people who buy your cart