I agree it's too long for an email, but it could be a reasonable length for a document that could avoid years of engineering costs. I'd still start with a TLDR section and maybe have a separate meeting to get everyone on the same page about what the main concerns are. People will spend hours talking about a single concern, so it's not like they didn't have the time, they just find it easier to speak than to read. But if the concerns are only raised verbally they're more likely to be forgotten, so not only was that time wasted, but you've gone ahead with the concerning proposal and incur the years of costs.

A hard fact I've learned is that even if people never read documents, it can be very helpful to have hard evidence that you wrote certain things and shared them ahead of time. It shifts the narrative from "you didn't anticipate or communicate this" to "we didn't read this" and nobody wants to admit that it was because it was too long, especially if it's well-written and clearly trying to avoid problems.

It's still better to make it shorter than not, but you also can't be blamed for being thorough and detailed within reason. I try to strike a balance where I get a few questions so I know where more detail was needed, rather than write so much that I never get any questions because nobody ever read it, but this depends just as much on the audience as the author.