Are you truly re-enacting Sisyphus's struggle if you don't use a chonk D20 and mini dice? Game is a fun little activity, but I have to wonder if there's any reason to not use D4/D6, D8/D10, and D10/D12 pairs?
Love the picture!! Single player definitely needs some retooling. The game shines as a multiplayer game, because you can use different combinations to set a better pace than your opponents. For single player, there is no reason not to “Work Harder” all the way to 20 and hope for a good roll. Maybe a turn counter would help? Let me know what you (and anyone reading this) think!