batch(1) is a little known posix command that supposedly takes a list of jobs/shell scripts and executes them when the load average of the computer drops below a certain level. It's far from an ideal scheduler, but it's builtin and does what is should do. Problem is, the queued commands actually do not execute sometimes. The load average limit is 1.5 by default, but I have 6 cores/12 threads, so my load average rises easily above that.

The atd(8) daemon is responsible for job execution. Add -l $(your_load_limit) to the atd invocation in /etc/systemd/system/multi-user.target.wants/atd.service (which is a symlink to /lib/systemd/system/atd.service on my system), or use the following shell commands to create a copy with the right configuration.

$ sed '/^ExecStart/s/$/ -l '$((`nproc`-1))/ /lib/systemd/system/atd.service | \
    sudo tee /etc/systemd/system/atd-replace.service
[Unit]
Description=Deferred execution scheduler
Documentation=man:atd(8)

[Service]
ExecStart=/usr/sbin/atd -f -l 11
IgnoreSIGPIPE=false

[Install]
WantedBy=multi-user.target
$ sudo systemctl stop atd.service
$ sudo systemctl disable atd.service
$ sudo systemctl enable atd-replace.service
$ sudo systemctl start atd-replace.service