If you're looking for an all-in-one Raspberry Pi handheld, you might appreciate the YARH.IO MKI project. This gadget is completely portable and self-described as a "hackable Linux handheld".
The YARH.IO can be customized with add on boards and adjusting the physical modules on the handheld. Certain components can be removed, like the keyboard. Because it relies on a Raspberry Pi, it supports a variety of Linux-based operating systems.
This edition is designed for the Raspberry Pi 3B+. It features a 5" touchscreen with a resolution of 800 x 480. There is a keyboard module as well as a touchpad for cursor control. It's completely portable, using a single Fenix ARB-L21-5000 5000mAh rechargeable battery that can be easily swapped for a fully charged one. It also features a DS3231 RTC clock module.
The GPIO connectors were made accessible through the bottom of the main module. This makes it possible to include any additional add-on boards. The shell used to house the components were digitally designed and 3D-printed.
You can find a list of components needed to recreate thus project yourself on the YARH.IO website. If you want to purchase a pre-made unit, you may be able to get you hands on one of the limited number of fully functional prototypes through the official shop. These currently cost $629.