The installer most definitely offer a choice of root file system - it defaults to btrfs - but the combination of the default file system compression (zstd) and the compressed init (also zstd) may create issues on some systems.
See → [root tip] [recovery] Basic Manjaro Linux Rescue and Recovery for instructions to mount the btrfs layout and chroot into the system.
As you are using btrfs - to avoid the conflict between file system compression and init compression you may change the compression for the init to cat
Then re-build your init
mkinitpio -P
To reinstall the grub boot loader simply run the manjaro grub installer script
install-grub