setting load limit for atd batch system?

So the atd supposedly will not start another job until load factor falls below a limit. Different documentation gives the default as 0.8 or 1.5
Now I launch a job that uses 4 cores on a 6-core CPU. If I run top command, I see four processes running close to 100%.
Now if I submit another job 10 seconds later, that starts thereby overloading the CPU. Documentation suggests setting load limit to more than n-1 for n CPU cores, but I think that is intended for single-thread jobs. I have tried altering the load limit in atd.service file to all sorts of values, but second job keeps starting while the first is flogging the CPU. I check with 'ps -ef|grep atd' to see it is using the desired load limit. I am aware that the load factor is an average, you can see it changes slowly in top/htop/glances. So I also increase the delay between jobs to 30 seconds, but still nothing works. So it looks like I have to specify a time like 'now+60 minutes' when I submit, requiring some guess how long first job runs. I know I can install a proper job scheduler such as Some Grid Engine, but that is more work.
This is on Debian 11, by the way.