May I ask if you have also tried, in that same /etc/default/grub file, to have GRUB_CMDLINE_LINUX_DEFAULT with NO options inside ? I mean, like the following:
GRUB_CMDLINE_LINUX_DEFAULT=""
(naturally, please run sudo update-grub after doing that change and then reboot).
I ask that because that's my preferred option and it seems to have helped other people, here in the "Ubuntu MATE Community", in different contexts. See, for instance, the following replies of mine in different topics:
