What do you mean by “soaking” by the way?
Alot of the warping comes from the aluminum platform deforming when it heats up, not necessarily just the heater or the sheet
Just want to confirm you have actually pre-heated the entire print bed via simply turning it on from the settings/jog screen.
lessening the calibration grid is not going to be helpful, the 5 x 5 is the best calibration the firmware offers via the display.
theres no question its tough to print large objects on the snapmaker 2 but what you are showing us and telling us - this should be able to be mitigated quite a bit at least.
Now, if you want to go to the next level… you can run a custom 11 x 11 calibration.