crond will start on boot if there is one task or more in the list. Cron is good for running commands that have an end, for example run a backup script on a schedule.
To keep a service running you should make a service, so an init script for it https://openwrt.org/docs/techref/initscripts
possibly one using procd functionality https://openwrt.org/docs/guide-developer/procd-init-script-example because procd is able to track the service status and restart it if crashes