Is there any software constraint on what is the largest HDD you can put into a rather old QNAP NAS?

I currently has 4x8TB HDDs in Raid 5 inside my NAS (TS-439-PROII, so pretty old model) at the moment. The offical compabilitity list of the HDD for the device only go up to 6TB but my units with 4x8TB has worked just fine for about +5 years now. I am thinking about upgrading the HDD to larger ones because of Linux ISOs.

I understand these larger say 18TB or 20TB HDDs are not officially supported since my NAS is so old so these are untested hardware, but is there a software reason why i couldn't replace by 8TB HDD with ones with much larger capacity ones? Could I somehow get one of these 18TB drives and test if it works or not with my NAS?