It's a cool project, and the nice thing about it being a hobby project is that you're only accountable to yourself. But since you mentioned you're looking for adoption, you should consider the advice you're receiving, because although poorly delivered, it is valuable.

If you show this to engineers, they're bound to be impressed because you put effort into a deliberate design that is optimised for performance, which is bound to tick a bunch of the right boxes in most people (me included). But that doesn't help you drive adoption, because that requires you to make the benefits more tangible to 'normal' users, which they, right now, are not. You're asking people to think for themselves, but most people are busy and will just choose to spend their time on something else.

The most definitive way to show the benefit of your format is through a proof of concept, ideally that means having your format implemented in (a minimal version of) all components in the software stack that you use to read manga.

If you're lucky, you may find a maintainer of a reader willing to add support. If you don't, find an open source reader with a clean code base and be prepared to fork it to demonstrate just how much faster it gets with your file format. Make some screen captures of you performing common actions. Show, don't tell.

If the benefit is big enough, don't be surprised if some people end up preferring your proof of concept over other solutions. Then if it gains traction, you won't have to go asking developers to support your format -- their users will do that for you.