I'm not entirely sure what the OP meant, but the scenario as described - "a standalone handheld that I can type in a message without T9-style pecking" - would be served perfectly fine by an on-screen touch keyboard.
And there already are Android-based phones with transceivers! E.g. Unihertz also has one: https://www.unihertz.com/products/atom-xl. So basically it just needs to be something similar, but with APRS.
And there already are Android-based phones with transceivers! E.g. Unihertz also has one: https://www.unihertz.com/products/atom-xl. So basically it just needs to be something similar, but with APRS.