I've decided to order a 2.8" 320 by 240 SPI display:
https://thepihut.com/products/2-8-touch ... co-320x240
This uses a similar SPI controller to the one I've been testing with and also has an SD Card built in. In theory it should be possible to create a self contained display and storage solution that performs like a UK / European ZX8x. There will be no sound, but I know some do not see that as a deal breaker. I'm planning to ignore the touch capability of the display for now.
As the frame rate will be effectively software controlled, it will be interesting to see how the frame rate effects that some ZX81 programs introduce appear.
I've got a few other projects on the go at the moment though, so it may take me a while to post the results.
The screen resolution is 320x240. I've set the LCD up to receive RGB444, so that it can work at a sustained 60 Hz refresh rate. The refresh rate can also be configured for 50 Hz and 50.65 Hz. The Waveshare board has an SD Card, and the Pico plugs straight into the back of the board, so it is a very compact solution.
A couple of images can be seen below. The first shows Hi-Res Galaxians. It can be controlled by either the joystick or the USB keyboard (made from a 3d printed ZX81 case) that are also in the shot. The second shows MaxDemo (just) filling the screen, although in retrospect the picture makes it look like the screen has cracked
As ever, more information, source code and executables etc can be found at: https://github.com/ikjordan/picozx81