If what you really need is security, then I don't see how (1) and (2) can be overriding concerns. If I were in a situation where my life depended on my messages being confidential, my choice of medium wouldn't turn on the convenience of long-form messages or social conventions around reply time.
To put it another way: if I'm in a situation where being able to type an easy-to-read long-form message is more important than it being end-to-end encrypted, I'd just use Gmail. I don't see why Signal needs to satisfy both use cases; at worst it's kind of ergonomically awkward, but it isn't making false promises of security, which is worse than useless.
To put it another way: if I'm in a situation where being able to type an easy-to-read long-form message is more important than it being end-to-end encrypted, I'd just use Gmail. I don't see why Signal needs to satisfy both use cases; at worst it's kind of ergonomically awkward, but it isn't making false promises of security, which is worse than useless.