Finally, after 5 months on the to-do list, I have finally got round to changing the default boot partition on my linux-MacOS dual-booted t2 macbook.
The guidance given at t2linux did not work.
Please note that I do have two separate EFI partitions. If you have only one, this will be more useful.

Here’s what I did, based on this post:

In linux:

  1. Use lsblk to find which is your boot partition. Mine is /dev/nvme0n1p3

    • note that below “nvme0n1p3” becomes “disk0s3”

  2. Shutdown

  3. Boot into MacOS recovery mode by holding cmd-r

In MacOS recovery mode:

  1. At the top bar, go to Utilities and open the Terminal

  2. mkdir /Volumes/EFI

  3. mount -t msdos /dev/disk0s3 /Volumes/EFI/

  4. bless --mount /Volumes/EFI --setBoot --file /Volumes/EFI/EFI/BOOT/BOOTX64.efi --shortform

  5. reboot

Note that you will have to do this each time you upgrade your MacOS version.