I’ve only been a couple times to Disney but both have been with kids (2 year old in diapers and then 5 year old and a 2 year old in diapers). I did a ton of research beforehand to be ready and I’ve also just in general had several diaper bags and even just purses and totes over the years. I absolutely agree that any backpack can be perfect, don’t limit yourself to a “diaper backpack.” In fact, some can tend to be very heavy empty, which is a negative, and expensive.
My tips: we found that having two bags was actually best (rather than one large backpack) if you have a stroller. A smaller backpack with the essentials: wallet, phone, any other irreplaceable items, some snacks, diapering needs in a pouch that you can grab out for easy diaper changes. And then an extra bag, like a cheap tote bag, with extra diapers, extra snacks, extra change of clothes, etc that just stays in the bottom of the stroller. I never worried about leaving extra stuff like that in the stroller but of course, don’t leave anything that you’d be distraught over if it did get lost or stolen (highly unlikely, but still). A smaller backpack is easier for rides and for wearing for longer periods of time than trying to carry everything. I’d also consider stroller hooks if you don’t have them already.
I love the Jujube Minibe, the smaller Vera Bradley backpacks, and the smaller Kipling backpacks for everyday smaller backpacks and for Disney. You can also check
Amazon for other budget friendly options.