salve,
a long side scx.service and scx_loader.service is systemd.path possible.
this scx.path will launch scx.servce when the necessary infrastructure is present
[Unit]
Description=starts scx.service
Before=shutdown.target
PartOf=scx.service
[Path]
PathExists=/sys/kernel/sched_ext
[Install]
WantedBy=multi-user.target