Over a week later, I can confirm that manually applying this fix and restarting the middlewared service has solved both bugs: nightly boot-pool scrubs and Scrub Tasks ignoring “threshold days”. 
Nine months of waiting for a fix that will never arrive, yet I could have applied this fix after 13.3’s release in September 2024. It took me less than a minute to do.
Wish I had known this was possible back then.
If anyone’s interested, all you need to do is use nano to edit the above file, add a few characters of text, and then restart the middlewared service. The fix will survive reboots.