I'm only assuming as the hardware package that comes with the stack includes a bunch of M3 screws and standoffs. But yeah, I'm seeing a lot of 20 X 20 boards that have M3 holes. I guess it's because they're advanced enough now to be utilized on full-sized frames.
I have the same board. Problem is that the 4 in 1 esc connects to the fc via pins rather than a typical 8 pin Jst 1.0 connection. The spacers they give you are 3mm plastic spacers to go in between the esc and fc. the grommets that RENOV8R is suggesting above are too long. I’m doing some blackbox logging to see what works best at vibration reduction on the board.
My thoughts are soft mounting the motor or o-rings. I did try similar grommets but found that I couldn’t screw down everything evenly so that the gyro was actually level