Isn't it a little weird to tell people to do major surgery on a crypto design, especially in ways that depart from the well-regarded design they derived it from, on account of attacks that don't work against that design?
(N.b. I don't expect my specific suggestions to be adopted. However, I view complaining without offering solutions to be poor form, so I offered some alongside my complaints.)
Of course, any deviation from Signal's design can and should be vetted by the same experts that vetted Signal's. And if they're unavailable to vet the derivations, the conservative thing to do is grit your teeth and bear Signal's legacy until they become available.
However, if Signal still targets Android 4.4 phones in 2020, there's a lot of devices without AES-NI and thus improving upon their AES-CBC design decision is worth probing at least.