ok - I’m done with a lot of experiments. Long story short, disabling the smartd service is what reliably lets my drives sleep in a timely manner. and as others have noted, setting the dameon’s Power Mode to “Standby” doesn’t help.
Plenty of details below for anyone interested 
I set APM on one half of the mirror to 64 and the other to 127 and they both went to sleep. As I’m still unclear what the behavioral difference is between both levels (I assume it has something to do with how aggressive the power curve is), so I set both to 127 to reduce variables. I want the drives to perform maximally when they are spun up and sleep when they’re supposed to sleep.
I reenabled S.M.A.R.T. on one drive and left S.M.A.R.T disabled on the other and left it overnight. Turns out that both drives end up going to sleep for 6 hours which is great news as I would prefer to leave S.M.A.R.T. enabled. The only reason they woke up was I went to reenable S.M.A.R.T. for the 2nd drive and 50 min later they went to sleep. Not sure what the +20 min (over 30 min) is all about but at least they are sleeping.
I flipped on the S.M.A.R.T. daemon (which also wakes up the drives) - the Check Interval is set to 30 and Power Mode is set to “Standby” - I don’t have any related scheduled S.M.A.R.T. tasks active at the moment. Consistent with what I hear in this thread, smartd (even set to “Standby”) doesn’t let the drives sleep after an hour passed. I turned the daemon off and sure enough, a drive goes to sleep after 30 min but one half of the mirror was still active! Not sure what was up with this, realized it seemed like the daemon was still active so I toggled it on and off and both drives went to sleep in 30 min - score!
So moving forward, my maintenance window will be at midnight on Sunday when snapshots happen and wake up the drives. I use a utility called TimeMachineEditor so two Macs fire off their backups shortly afterwards at 12:05a and 12:10a respectively. I sent a note to the developer to see if multiple TM destinations can be configured with different schedules (as I have another destination off the NAS I would reserve for daily backup)
Prior to all the testing, my system pool was moved to my Boot Pool (128GB NVMe boot drive). I have 8GB RAM I plan to upgrade to 32GB. Since my loads are primarily read-only, I set up a spare NVMe as a L2ARC to see what happens (32MB of my LARC is indexing 50GB at the moment). I have no app pool activity as I haven’t set up any apps yet and will ensure this gets put on solid state.
Ok hope this helps someone demystify their config out there!