Btw. I made a pull request to support the Acer Predator W6d and it is not necessary to dd the kernel partitions. You could just invalidate both boot slots with setenv dual_boot.slot_0_invalid 1 and setenv dual_boot.slot_1_invalid 1. But in my u-boot version a failsafe web ui is then started, so it is better to change the bootmenu_default to one bigger as you have entries. For example
bootmenu_0=Startup system (Default)=mtkboardboot
bootmenu_1=Upgrade firmware=mtkupgrade fw
bootmenu_2=Upgrade ATF BL2=mtkupgrade bl2
bootmenu_3=Upgrade ATF FIP=mtkupgrade fip
bootmenu_4=Upgrade U-boot only=mtkupgrade bl33
bootmenu_5=Upgrade partition table=mtkupgrade gpt
bootmenu_6=Upgrade single image=mtkupgrade simg
bootmenu_7=Load image=mtkload
bootmenu_8=Start Web failsafe=httpd
So setenv bootmenu_default 9 worked.