apt install dropbear-initramfs

/etc/dropbear-initramfs/config
DROPBEAR_OPTIONS="-p611 -s -j -k -I 60"

# Add RSA keys here (no ed25519 ?)
vim /etc/dropbear-initramfs/authorized_keys

# update initramfs
update-initramfs -u

# Set up grub
vim /etc/default/grub
# Like:
GRUB_CMDLINE_LINUX_DEFAULT="ip=client-ip::gw-ip:netmask"
# Example:
GRUB_CMDLINE_LINUX_DEFAULT="ip=10.1.1.100::10.1.1.1:255.255.255.0"

# update grub
update-grub

# Set up ~/.ssh/config for client to connect to dropbear/ssh:
Host sf-001-disk
    Hostname 10.1.1.100
    Port 611
    User root
    PasswordAuthentication no
    IdentityFile ~/.ssh/id_rsa

# Then when system boots up, ssh to it:
ssh sf-001-disk

# When logged in, run:
cryptroot-unlock