My current "mobile typewriter" is a Boyue Likebook Mars with a Fujitsu HHKB BT:
https://ibb.co/hWPXmCt
I have a few quibbles with the device as an eReader, but for every other use it's been freaking incredible. Running Bluetooth on the tablet definitely drops the battery life, but I can still get a full days use out of it and have some battery leftover by the time I plug it back in.