Ok, I've done the so called "manual (factory) reset" and now let's see how the battery drain in sleep mode behaves after that and with an older firmware. After doing that is says it is now on firmware version: 4.34.
20148 (57c2aebbb6, 10/6/22). Until this moment I've used it with latest firmware (4.38.xxx) installed.
As far as I can tell that firmware build wasn't released as an update?
Any suggestions in regards to recommended firmware build (other than the latest, 4.38... since that one was on my device for previos 45 days and poor battery life I've observed was with the latest firmware) to try? 
What I've done is: factory reset from Settings menu > power off > manual factory reset> choosing "I have no WiFi" option to let me connect to my PC to edit "Kobo eReader.conf" file to add "SideloadedMode=true" line > transfer a single epub file using Windows Exploer (so no Calibre, no Kobo Desktop app thing) > disconnected USB cable and let it import the added book > forced a power off (to "apply" / to turn on SideloadedMode),since it was only offering WiFi setup or no WiFi thing, still even after editing the config file > once it booted I've turned off WiFi, BT, sleep cover wake up option.
Now charging it back to 100% and will not be using the device at all for next couple of days.