The Snapmaker 2.0 printing bed is unfortunately infamous for being bumpy - some people have worse, some have better print beds. There are several looooong thread in the forum - this perhaps being a good one to go down that rabbit hole: Bed Leveling - Revisited & Detailed
I do have a nasty, bumpy bed - I’ve got the Kickstarter machine, and those were especially bad. Still, for me with the dual extruder the bed levelling with the densest pattern (I think it was 5x5 - or was it 7x7? Sorry, currently have a job running on my machine, cannot look) yielded good results, but I yet did not print large things with the DX. With the single extruder, I did 11x11 patterns (you need to do that via GCode, not via Touchscreen) and was able to get a really huge good print: Maximum Y Position 347mm? - #9 by Hauke. The DX unfortunately uses a different levelling procedure, and I never tried if via GCode you can go for larger patterns.
What worries me most is: With the single extruder Snapmaker uses standard marlin methods, which means that the bed levelling is not only used for the first layer, but for subsequent also. There’s an option to slowly “phase out” the levelling layer by layer. For the DX, with Snapmaker’s self-made, homebrew levelling I would not think it impossible that they only apply levelling data to the first layer, which would explain your problem… perhaps Snapmaker support can help here? If they really messed this up, the community should push hard on them to use the Marlin method also for the DX!