$ doas sdparm --command=eject /dev/sdb
    /dev/sdb: Seagate   BUP Slim SL       0302
$ doas sdparm --command=stop /dev/sdb
    /dev/sdb: Seagate   BUP Slim SL       0302

When I use this combination of commands my Seagate USB hard drive appears to power down, the power LED indicator goes dark.  I have also tried the same commands with a Sandisk USB memory stick that also has an LED indicator, but in that case the LED stays illuminated.

If you have interest: It would be surely also possible to look into the code for udisks2, therefore also the call-stack of the command (program) and trying out to find possible ways forward. If you want a command from udisks2 you need to use that package / software or replicate it that near so you can have a comparable outcome. What we have learned since the release of Hyperbola 0.4: There are commands and packages not being possible to be replaced or imitated. They are Linux-only (without GNU also) and not available elsewhere. The command udisksctl is the udisks command line tool, only available there. Imitating or replicating it, is only possible to either research or rewrite ... or replace it with different sets of commands, different and more perhaps, but nearing.

I keep this thread so far open: Perhaps others have a different perspective on that matter or another solution? Eager and interested to hear and read about.

sync
umount

Those should be sufficient as udisks2 is just another layer above without a current need doing so. You can also use pmount in combination with spacefm for example when you want a graphical interface for the file-management. But in general as said: udisks2 is exactly the opposite point of what Hyperbola wants to offer and therefore also not available as it offers technical seen no further need or enhanced usage.