To install a solid Ubuntu with working grub, mouse, touchpad and wi-fi on MacBook with said configuration I have used a distribution from GitHub repo, where Ubuntu versions for T2 MacBook are posted. I've installed release v20.04-5.7.19-1 (download an ISO image, write it on USB stick with balenaEtcher, create a new disk partition for second OS, enable booting from external media, then run Ubuntu Live and use Ubiquity to install).To enable wi-fi I followed steps proposed AdityaGarg8:install necessary kernel modules with DKMSupgrade kernel from v5.13.12-1follow this guide
After that I have Ubuntu running without problem on Mac with configuration that described in the question.
Try this:Hold cmd-R while booting and type your user password when prompted.Open the "Startup Security Utility" from the Utilities menu.Set Secure Boot to "No security" and Allowed Boot Media to "Allow booting from external or removable media".Restart holding option and select USB drive.
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