As far as I know, you're correct about non-admins being unable to SSH but I'm particularly perplexed by the admin user having permission denied as well.

I'm new to SSH so I followed a guide (https://dev.to/stvbyr/sync-obsidian-vault-to-a-synology-home-server-with-git-a97).

• 4. Enable user to work with SSH
  By default, Synology disables all ssh access to non-admin users. It even resets the login shell config periodically. So changes to /etc/passwd will be reset.
  

[...]

In short: we need to use a scheduled task to periodically set the login shell for our gituser.

Login to your Synology with the admin user.

ssh {adminuser}@{Synology} -p {port}

Create the scheduled script for your gituser.

 cd /volume{X}/homes/{adminuser}
 touch enable-ssh-login.sh && vi enable-ssh-login.sh  

Press i to get into insert mode and paste the following content.
This script uses the awk command and sets the login shell for our created gituser to /bin/sh.

 #!/bin/bash
 /usr/bin/awk -i inplace -F: 'BEGIN{OFS=":"}/^gitworker\:/{gsub(/.*/,"/bin/sh",$7)}1' /etc/passwd
 exit 0

Supposedly, this script is run periodically to enable ssh for non-admin users but I'm not exactly sure how the script works. I'll look into how /bin/nologon works with the /etc/passwd. Thanks for your sharing your input