I can’t help with that particular issue, but I was able to level test using a single 100mm x 100mm x 0.1mm test using Snapmaker3D. I haven’t really played with SnapmakerJS yet. I didn’t try scaling it up to 120mm x 120mm. I was able to generate the GCode in Cura, but I didn’t print it.
For level testing, I found a 0.1mm thick print to be more useful. I saw one corner was noticeably thin compared to the other corners, even though the calibration paper it felt the same to me. A single 0.05mm step was enough to fix that issue. That small of a difference might be obscured in a 0.2mm thick print.
I printed https://www.thingiverse.com/thing:2125379 . I wanted the full square, not just the outer rim. I was having problems with prints that would adhere to the bed, then get pulled off on future passes. I did abort plenty of tests after it printed the inner and outer walls though. It turns out that I had the head too close to the bed, and PLA was squishing out sideways, pushing previous passes up off the bed.
I was surprised at how much I had to modify the calibration to get a consistently thick 0.1mm print that didn’t get pushed off the bed. I re-checked the calibration paper afterward, and all 4 corners have a different amount of force required to move the paper. One corner is so loose that I can easily move the paper in all four directions, and the opposite corner requires a firm grip on the paper to pull it out. That was done after the print, so the bed was warm (but cooling off).