If I’m not wrong, currently all your servers are running under a single user account sysadmin. But you wish to create individual credentials for developers.

To achieve this, create individual users with useradd and set up SSH key based authentication.

Edit the /etc/ssh/sshd_config file to set AllowUsers <userlist> replace with developers usernames.

After editing restart the SSH services with Sudo systemctl restart sshd

NOTE:
Before restricting sysadmin make sure you have another account with sudo privileges.

For more information refer to this blog by tenable.

For Activity logging :

The auditd tool on centos can log user activities like file access, modifications and login attempts.

As per this Digital ocean tutorial by Veena k john and Tammy fox, the main configuration file for auditd is /etc/audit/auditd.conf. This file consists of configuration parameters that include where to log events, how to deal with full disks, and log rotation.

Since you have 70 servers, consider centralizing logs for easy monitoring. Use the Google cloud Logging and Install the Google cloud logging agent on each server. You can configure the agent to send logs from /var/log/secure and auditd logs to cloud logging.