Why there are no resources (RAM and CPU) dedicated only for administrators' access?

I have never had a system so busy I could not ssh into it. I have had systems that oom, and kill ssh, and other useful services. You should be able to use your cpu at 100 percent, and still ssh in. I would try to investigate exactly what is happening when your system because unresponsive. Maybe the logs will have answers for you.

I have never had to use it but ulimit is what you want. Some distros have a admin group and others don't. Put your none admin user in a group, and limit their resources accordingly. https://www.geeksforgeeks.org/ulimit-soft-limits-and-hard-limits-in-linux/