First time I'm seeing cron as a separate service. That's fascinating! I had to add cronjob to the host machine to get that to work. Can you explain more how that works? It kind of looks like you're running two nextcloud containers? I could be mistaken.
Not OP but I'm doing almost the same thing in Kubernetes. Basically you start a Nextcloud container but only to run the cronjob, not Nextcloud itself. In my case, Kubernetes creates a new container for each cron execution. Apparently there's also a cron.sh script already bundled with the Nextcloud image, that can run continously. At least OP doesn't seem to mount the script from somewhere else.