I found https://www.clockworkpi.com/uconsole (btw, any opinions about it?), and it seems it would be exactly what I want, but are there other options?
The only criterion you have specified that discounts about 98-99% of today's smartphones is "An important thing is that it should run neither Android nor iOS, so that there won't be a possibility of distractions like games."
That's easily fixed. You just delete the games.
https://www.aliexpress.us/item/3256805342629426.html?gateway...