Thanks to @kensanata I recently found an RPG system that looks really neat. It's called 'Dead Simple' and the 8th "expanded" edition is 20 pages. It uses a d20 and looks very playable. The site offer different settings, apart from pure fantasy, as well.
There is also a 2d6 version, Lamplight 3rd ed.
I haven't tried it yet, but this is likely the one I'm going to use in my new "pocket hobby project".
Only thing with Lamplight is you are to roll low, which feels a bit like 'bad mojo' for an old Classic Traveller (CT) GM/player like me. 🙃
Another person that has inspired me with their #dungeon23 project is @ordinal . So as part of this project, much of my ideas and concept work will be on paper. I have a thin Moleskine Cahier Journal with squared pages and a 2.5mm 3B Koh-I-Noor 5905 clutch pencil as my main tools, things that I always have me anyway.