An easier variant would be to simply play the game for X rounds, without a dummy player. I just do that, playing to 11 rounds, discarding from a pile of 11 unused cards as a timer.
That's a great idea - I never thought of playing it that way. I've been searching for a super simple (quick to setup, low rules, doesn't take forever to play) solo worker placement game I can relax to that doesn't have external pressures, and it turns out one was sitting in my cupboard all this time.
Just out of curiosity, you don't happen to have any other similar games you do that to?