[wrxtasy]

Enough processing power to run Kodi smoothly and play 4K 10bit HDR content without hiccups

And this is where the wheels fall off the wagon...you are too early for bugs free HDR support if using LibreELEC / Linux. Stick with the Shield.
You have to use Mediacodec (Surface) with Android and even then there are HDR bugs and issues like no Auto Resolution switching.

Connect USB HDD to the device and share the content to other devices on the network (having a 2.5 inch HDD which draws power from the USB port on the device would be preferred)

Strongly suggest a Linux / LibreELEC box here with its built in SMB server. Android permission are not much fun to deal with compared to the simplicity that is LibreELEC. It is virtually Plug N Play with external USB storage.
LibreELEC's SMB server is already setup for sharing files.

A good and simple RF/BT remote (I love the small nvidia shield remote). Also would be great if the remote had keyboard on the back)

MINIX has that with their NEO A2 lite.

Possible options on my radar so far:
Vero 4k
Minix Neo U9 / U9-H
Xiaomi Mi Box

Vero 4K - like LibreELEC no proper HDR support with OSMC. No Keyboard remote like the U9. Otherwise a Solid choice.

The U9, whilst good as a touchscreen/tablet Android device, comes with the usual Android limitations mentioned above. HDR has issues as well.
The U9 does not run LibreELEC properly, there are GPU video driver issues.
Why would you buy another Android device when you already have a Shield ?

You will not get Kodi dynamic refresh switching with the Mi Box, so no ultra smooth video playback. 4K playback not as good as an OSMC / LibreELEC box, plus the usual Android limitations.

The trouble is trying to do everything on the One device so you will have to make compromises.
Use Android for Android Apps and 4K HDR, leave it in a 4K resolution if you have to. Put up with poor video quality upscaling from the media player.

Android really does not play nice with 4K TV's due to the lack of Auto Resolution switching from 1080p <<-->> 4K. If you value quality 720/1080p video upscaling on a 4K TV, do not use Android for a 720/1080/2160p (non HDR) Kodi media player. Really a two player solution is needed.

The S905 MINIX U1 running LibreELEC with its included quality A2 Lite Mini Keyboard remote will come the closest to your needs. Ditch HDR.

Otherwise an AML S905X device setup as a dual boot LibreELEC / Android (HDR experimental) device. Android will not be fun on this type of device, there will be numerous issues. Buy a MINIX A2 Lite remote for it.