Set pub key login only for ssh:
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First, you will need to create the key pair and place correct files in the ~/.ssh older for your user on both the server and each client.

Secondly on Ubuntu delete these secondary config files that can mess up the main config file:
sudo rm ssh_config
sudo rm ssh_config.d
sudo rm -r ssh_config.d

Edit the main config file:
sudo nano sshd_config

Replace the entire file with these contents:

# This is the sshd server system-wide configuration file.
# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# Port and ListenAddress options are not used when sshd is socket-activated,
# which is now the default in Ubuntu.  See sshd_config(5) and
# /usr/share/doc/openssh-server/README.Debian.gz for details.

#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

PermitRootLogin no
PubkeyAuthentication yes
PasswordAuthentication no
PermitEmptyPasswords no
AuthenticationMethods publickey
KbdInteractiveAuthentication no
UsePAM yes
X11Forwarding yes
PrintMotd no

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*


Save using Ctrl+X

Run:
sudo systemctl reload ssh

You should now have pubkey only auth!

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Sources:
https://wiki.archlinux.org/title/OpenSSH#Force_public_key_authentication
https://branden-g.ddns.net/Notes/Tech_Help/Linux_Repair/Linux_Settings_Changes/Ubuntu_Server/Disable-root-login-for-SSH-on-Ubuntu.txt

Date: April 19th 2024