Cage of Sand is a solo (or optionally multiplayer) tarot-based timeloop horror game. It's short, has a stark but lovely layout, and comes with a number of safety tools.
It *is* very player-GM'd, and a lot of its mechanics run on interpretation of prompts, so if you're not comfortable telling a lot of your story yourself, you might want to modify some mechanics and appoint a GM.
Overall, this is definitely worth checking out if you enjoy timeloop fiction such as Re: Zero, Tragedy Looper, certain stories about cicadas, Happy Death Day, etc. If you haven't tried timeloop stories yet, or haven't hit one you really enjoy, this is still absolutely a good starting point if you also enjoy horror.
-I'm not sure I completely understood the 54321 mechanic. Is it a grounding exercise? An automatic break in gameflow to cut the intensity? That seemed to be the intent, but I didn't spot an explanation outright.