Just sharing some information I’ve found on the topic.
@Tone posted some sample gcode on how he did it here: Check Level Hot.gcode
He also has this sheet with information on how the auto levelling process works under the hood, it’s a custom wrapper around G29, and you can’t just call G29 or, like you found out, it’ll crash. How to do a larger than 3 x 3 matrix level
I took a 10x10 grid using a micrometer at both ambient and at 60C, and which the movement wasn’t much, the center of the bed raised 0.05mm and the corners moved more. I’ll try and format my findings later and update this.
I talked with someone who suggested using a macro to use the built in G30 single point probe in a grid, but turning the bed off immediately before taking the reading, as the electrical current in the heater can mess with the induction probe’s reading. I plan on trying that too, as I have noticed the bed moves a decent amount as it heats and cools, and it would allow for taking more readings at a more constant bed temp.