The SSDs on the new iMacs absolutely have full S.M.A.R.T. support. Disregard what System Profiler may or may not say about it.
The problem is that most of the current S.M.A.R.T. utilities (and Smartmontool) apparently do not yet have support for NVME.
DriveDX has a beta with NVME support which I am currently using. If you own DriveDX you can write to their customer support about it and they'll send you a link.
OWC has a program in beta called
Smart Alec that also supports NVME SSDs which you could sign up for but it isn't nearly as elegant as DriveDX.