If you already completed the First Kiln, here's how to unlock the new game modes without doing it again
First, close the game and find your user data folder. Depending on your platform, type this into the address bar of your file explorer:
Windows: %AppData%\Godot\app_userdata\Idols of Ash Linux: ~/.local/share/godot/app_userdata/Idols of Ash
Then find the player_data.cfg file and edit it using a text editor. Change this line and set the difficulty to 3
[unlocked] difficulty=0
Then run the game, sorry for the hassle!