Anyway to get sig in callback function?

I don’t believe this is possible because the cron event runs asynchronously. In addition, the event that actually runs will have a different signature because it runs a clone of the event with a different timestamp.

The way that WordPress core runs cron events also means that the event gets removed from the cron events array just before its action fires, so there won’t be an event to reference by the time your event callback runs. Ref: https://github.com/WordPress/wordpress-develop/blob/7d5b488b3b8cbca0b0b8d979175a57eca38c7807/src/wp-cron.php#L127-L138