Civilization 6 turn timer for single player

They haven't had single player turn timers in the Civilization series for a while now. Your only options for single player turn limiting is to set up the Score Victory or Turn Limit options.

Score Victory - At the end, highest score wins.

Turn Limit - After a set amount of turns 200, 250 (whatever), the game ends.

As you mentioned, turn timers exist in Multiplayer. I'm not sure if this works, you'd have to test it:

You could try to set up a multiplayer game privately, add a bunch of AI and yourself, and start the game without any other human players.