ok. I report my situation.
I did everything suggested, twice, complete with factory reset and reinstallation from lgup.
Then, I installed all my usually used apps and then waited to see. I just gave permissions to the most important apps, and reduced synchronizations to minimum required.
Battery started to last very long and I thought the problem was solved some way. I could do 24h of battery use, with android system consumption being just 9% and with 4 to 5.30h of active screen.
Then, since two days, the issue magically came out again. Today, my phone has been active from 10:00 am and now it's already 33% with JUST 2h of active screen and 21% of android system drain. No big apps' drain from play store apps.
So, there must be something that works in background, something that can't be solved just by factory reset. It's something in the way Nougat works.
I think I will go back to MM.