Hello @Swdfm thank you very much for your reply. In terms of having a TM (/ book/ scroll as it is called on the game), it may be difficult to find a place. However, there are certain Pokemon that can learn Flash through leveling up. They are:
Reference page for info: https://bulbapedia.bulbagarden.net/wiki/Flash_(move)
1) Morelull (gen 7 learnset, at level 8); Morelull as we know is obtainable at Toadstall Woods, quite early in the game. If it's learnset can be edited, that's an option.
2) Shiinotic (evolved form of Morelull) --> I actually evolved Morelull to Shiinotic (at level 24) and tried visiting the HeartScale Move Relearner at Oregano City, but even that option did not work out (given that as per gen 7 learnset, Shiinotic learns it a level 1)
3) Kadabra and Alakazam, in Pokemon Let's go Eevee/ Let's go Pikachu can learn it at at level 1/ 13; (Side note: Alakazam should be obtainable via trade stone on Kadabra)
These are some potential options in my view.
I totally appreciate this is your game and you'd like the element of limited lighting to present a challenge, another suggestion (if you don't mind) is just to discard the limited illumination in the NORMAL difficulty mode and retain it in the HARD mode.
Based on the above suggestions, I hope you can appreciate you much I'm enjoying the game! Amazing work done in creating this game, thanks a lot!
Cheers