I have never used TrueNAS but I guess you could provision a separate volume and use iSCSI on top of that, and serve your very large database just to make things easier.
you can in theory connect multiple hosts to the same JBOD with SAS, there are even "SAS Switches" that are built for that purpose.
But if you do this incorrectly and initiate the incorrect drives your TrueNAS data is bye-bye
not sure I understand, what OS should these computers run? should this also be NAS storage? Or is it just the database filesystem that should be on the NAS?
honestly it looks like you don't have the knowhow to setup shared storage, it seems you are more a developer type of person, so KISS (Keep It Simple Stupid)
run local storage, your databases won't be even gigabytes in size, get some cheap SSD drives instead (SATA) if you want I/O.
also all your databased won't dive when your accidentally pull the power plug to your NAS or trip over the ethernet cable.