I would totally love to see more underground cheats if those are even possible. Perma 40/40 Diglett would be really nice, or timer doesn't decrease, or something. I haven't uncovered 100% yet but I'm spending more time down there post-game and it is quite a grind!You literally just run around and find either a dugtrio or diglett and walk over where it is. The ramsaes shards don't show up until the post game but you can get diglett shards from the beginning of the game when you first get the Explorer kit.
Its farmable by finding a short corner walk from entering and exiting the main habitat rooms. I farm underneath ramsas but you just check corners randomly and sometimes the diglett or dugtrio will be there. When you hit 40/40 a timer will start with a "SOMETHING GOOD MIGHT HAPPEN IN THE UNDERGROUND" text above the shard bar on the screen and every wall you dig up will have either a mysterious shard/fragment or a shiny pokemon statue. I think its a 2 minute timer and then it resets to 0 again and its very tedious to farm the 2 close diglett spawns I found. The timer still counts down while you are digging.
The higher the shard number, the better items you find. if you have less than 30, things like claw fossils and helix fossils won't spawn in walls and I didn't find a skull fossil for cranidos until the post game.