Thanks… turns out I should know how to do this already since I had to do it not long ago on an X1.
For future reference though… how do I get access to a terminal when its like this? As I mentioned there is no success with TTY and chrooting with manjaro-chroot -a gives grub-prob: error: cannot find a GRUB drive for /dev/sda1. Check your device.map. => No linux partitions detected.
I’m going to reinstall a third time and fiddle with the firmware manager - it was a real pain last time!
update: reinstalled and updated firmware, prompted to restart and now back to the original issue.
Manual chrooting with this guide: [HowTo] Chroot from or into any Linux distribution is not straightforward. After mounting mount /dev/sda /chroot contains boot/, boot.catalog, efi/, efi.img, manjaro/ and there are no system directories to bind to.