how to disable autoboot to clean keyboards?

There is ample time from when you open the lid until the login screen appears to make multiple swipes of the keyboard using a microfiber cloth — if dust is the concern. If the keys really need cleaning with a slightly damp cloth, then you may need to do this across a couple of boot cycles.

I have a 16-in 2021 MBP M1 Pro and that is my procedure, aside from washing my hands after meals and snacks where finger content would be transferred to the keys.

I am not aware of any way to disable this very annoying and unnecessary feature. You can provide Apple with product feedback here:

Product Feedback - Apple

One suggestion has been to invoke Startup Manager, aka Boot Picker, which uses the mouse to make its selections.

Intel: hold the Alt/Option key at startup.

Apple-Silicon: hold the power button st startup.

Change your Mac startup disk - Apple Support

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