Hello again Byrek!!!
Thank you so much for taking the time to both write out such a detailed response/request and giving such good ideas! I tried to implement Notation in a way that I think makes sense, I don't think it's the exact same way you recommend just because I thought this was simpler to implement for now. To use Notation press A+B at the same time (like you recommended! I wouldn't have thought of this so thank you). than you can use the D-Pad and the A button to navigate on a smaller 3x3 grid and show/hide Notation Numbers, then use the B button the exit Notation mode and go back to normal. This is the first iteration of this feature for now, and it doesn't look perfect because the boxes are really small so its hard to fit 9 numbers into them, but I figured this was better than nothing for now. I tried to find a smaller font like you recommended based on your measurement but I couldn't find any that actually fit 3x3 numbers in the small boxes. this was the smallest font I could fine for now (its just one that comes with the Playdate).I tried to make my own font for this but I'm terrible at design so I thought this was the best option for now. I also haven't added the automatic Notation removing yet just because of the you have to select numbers by cycling through them. I did make sure the Notation are save and loaded with the rest of the game, and I made sure the Notations are stored even after you select a number for the specific cell like you recommend.
if you want, let me know what you think of the current version! I know its definitely missing some pretty obvious features (like the auto removing of Notations) and the notations dont fit nearly as well as I would like but I think its better than no notations for now. Thanks again for all your input.