Good luck with the adventure keep us posted on what you uncover with cgroups
For the systemmd approach, you could do explore something along these lines*
[Unit]
Description=Set Playback daemons's CPU tweaks
Wants=mpd.service
After=mpd.service
PartOf=mpd.service
[Service]
Type=simple
User=root
Group=root
ExecStart=/usr/local/bin/set_cpu_shield.sh mpd
[Install]
WantedBy=multi-user.target
*Just a rough idea, please check if it works, and then if you have time, check if it follows “best practices”