When I tried RISC OS' own instructions to have the equivalent experience with their latest OS versions, they seemed to be missing a step, too, as I'd still boot to a desktop on a Pi 4: https://www.riscosopen.org/wiki/documentation/show/Raspberry...
Write the latest full RISC OS (through Raspberry Pi Imager or elsewhere). Edit CMDLINE.TXT per their instructions, and after it boots, on your keyboard, press F12 to open a command line at the bottom of the screen, type in the commands from their instructions, and then also type:
Unplug Desktop
In swapping the SD card between a Pi 1/2 and a Pi 4, I was getting occasional misfires, though, and it wouldn't boot into BASIC, getting hung up on the wrong networking driver, or complaining about a missing desktop. Ctrl-Break to reboot should fix it.
https://www.bencollier.info/projects/electronics/emulation/f...
My guess is that the RISC OS version of BBC BASIC will also have slightly better compatibility with things like old printed program listings than BBC BASIC for Linux, so may be a better choice for the full nostalgia hit.
RISC OS Pico [1] (I think it was short for Pi Computer) booted the Pi directly into BBC BASIC. It would be great if they'd make it a supported image on the current Pi boards.
The .zip archive is still available if you follow the links on archive.org (and even includes the book "First Steps in Programming RISC OS Computers" and associated demo programs.) Its 2.1MB boot .IMG is somewhat smaller than Ubuntu. Has anyone tried it?
[1] https://web.archive.org/web/20181109020010/https://www.risco...