I challenge your assertion that all home NAS devices..
No such assertion to challenge.
...a problem with nearly all of the small "home sized" NAS units that I have seen, is they simply don't keep the drives cool enough. Fan or no fan, the airflow design is nearly non-existent.
Exactly.
But on a somewhat related note: what does the drive firmware (S.M.A.R.T.) show as the maximum (ever) recorded temperature for those drives? That's the number you should be most interested in. Drives sitting idle don't get nearly as hot as those under heavy load stress. If that's also still good and low for your drives, then no big worries there.
Cheers