New in the package is to check and spin down the HDD
To detect the hdd state - at least for the 2.5" Seagate 5TB drive, we need also an addition in the boot commandline. So either do this by hand or just flash a new image completely.
While the spindown seems to work quite ok, the spin down detection seems not to work with every harddisk. If so, the ui will never show the spinning disk icon… Let me know, if it works for you 
Could you advise what exactly to add where? I am not sure, though, if I should do so, because I have already configured the harddisk itself to spin down (and it does successfully), so not sure if this addition might interfere with the internal logic.
Aside from now starting hd-idle from withing syncosync (which is not configurable) and showing a nice rotating disk icon, there are two changes necessary on your existing raspios:
sed -i '$s/$/ usb-storage.quirks=0bc2:ac25:/' /boot/cmdline.txt
adds the usb-storage.quirks, to your kernel commandline
systemctl disable udisks2
stops udisks2 for checking smart values every 10 minutes…
Your harddisk should have the above usb-id (0bc2:ac25), you could check that with lsusb. After reboot, hdparm -C /dev/sda should work. Before the kernel line, it did not work. At least for this HDD…
When hdparm -C … does not work at all, still hd-idle should set the disk to sleep after 10min. But you won’t see the nice rotating disk 
Sorry for replying so late. Since my setup is working perfectly as is, I will not make any of these changes. I hope it cannot lead to problems in the future in the sense that my setup is not “as usual”.
I guess, hdparm -C /dev/sda will not work and so you will not see the spinning disk icon and get no log entries, that the disk is up and down. But I guess the disk will still be set to sleep, so everything else should be fine.