Hi splunko,
Good idea to have UPS on your Yunohost!
To get priorities right:
- The UPS does deliver power to Yunohost when you disconnect the wall plug, to simulate power outage?
- NUT does not provide information about state of the UPS, so you have to guess whether batteries are full and whether or not there is a power outage?
Did you find the not-mentioned model on the NUT-compatibility list?
Your opening post is elaborate enough, but I have some trouble following what I am reading. On top of your post are two links, but I can’t match the guides there to the steps you followed.
Anyway… Did you notice > at the end of
With some luck there is some information about what the problem is behind >. You can press the arrow butten → to show the rest of the line. The mentioned nutdev1 is sparky?
Yes, good find 
Now root can make changes (and read), members of the group nut can read the files. That seems ok, so I don’t think the problem is in your step 7. If you undo the changes in step 6 and restart nut-server, does it give the current error, or the previous ‘no problem’ result?
My thought is: there are two components, so probably one connects to the UPS, the other monitors the driver. As long as you don’t tell the monitor what to monitor, all is fine. After you tell it to monitor something, that something has to have the right configuration. If not, errors follow.
I searched for the vendor/product ID’s, and found [HCL] BlueWalker / PowerWalker VI 650/850 SHL (2018+ Models) supported by usbhid-ups · Issue #646 · networkupstools/nut · GitHub ; could that be of any help? The issue was closed over a year ago, but with Yunohost running on Debian 11, the issue might still exist in NUT that is installed.