[Chris_Newton]

The 400 isn’t quite what I had in mind. I was thinking more of a device usable even by the under-10 group, with a simpler and more robust physical design and, crucially, the kind of instant-on, guided programming environment the early home “PCs” had.

With early Sinclair systems, each key on the keyboard would act like modern auto-complete: you would start typing and keywords would appear almost magically. On the BBC Micro, you powered on and heard the trademark beeeeep-beep and were instantly presented with a prompt where you could start typing a program or other commands. Everything in those systems was geared towards immediate responses and inviting you to start programming them straight away.

[MrGilbert]

I think I know what you are after - I wonder how much a TI-Nspire CX II-T could fill the gap, allowing for on-device python programming since 2020.

OTH... Taking a RPI400, and build a stripped-down linux system which resembles Sinclair BASIC could prove to be a fun exercise.

Edit: I just realized how expensive the ZX81 was here in Germany back in 1981 - it'd set you back 450€ by todays value.