The thick and think combination terminology of vol and lun can be very tricky and has been a matter of confusion historically. Therefore, NetApp has recommendations when it comes to this combination.
Thick LUN behind Thin vol has no bearing on the containing aggregate. Which means when you "enable space reservation" on LUN, it only logically drops space from the volume (Book keeping), however the containing aggregate has no effect b'cos the underlying VOL is thin. The only time you will see drop in aggregate space is when you actually write to the lun.
Note: The writes to LUN (even with space-reserved) are actually not guaranteed (write operations can fail if containing aggr runs out of space).
In such scenarios, keeping an eye on the vol is futile, rather you should enable Autogrow for the volume and set the maximum size for the volume to the size of the aggregate. In this configuration, you just need to "keep an eye on the target aggregate free space actively", but you do not need to monitor the free space in the volume.
Reference for recommendations: