I have thought through tough problems using pen and paper. Have tried a lot of iterations of digital notebooks, but lost the fluidity and malleability of paper.
These days I have found the iPad + Pencil with an infinite canvas to be the best fit. I use Ahmni (https://www.ahmni.app/) which is a delight.
It did happen in the Muslim world. There are "slave" dynasties in many parts of that world (Egypt, India) which were robust dynasties borne out of freed slaves.
I made a music app for iOS for myself, after not finding anything that did what I wanted as a non-streamer: visualization, audiobooks support, section loops, bookmarks, sleep timers, import from any source, etc.
In the end, I did put it on the App Store (after months of cleanup!) but it's easily the most used app on my phone.
I made a Music app for iOS that is geared towards offline libraries and has features like visualizer, EQ, sleep timer that I couldn't find anywhere else.
These days I have found the iPad + Pencil with an infinite canvas to be the best fit. I use Ahmni (https://www.ahmni.app/) which is a delight.