I have similar issues, my phone used to last over 2 days but now it doesn't. Also, the phone gets so hot, it thermal throttles. It also came to the point where the actual lettering of the Nokia logo came off. Android 8.1 made this issue slightly better, but it's still an issue. I'm hoping Android 9 Pie solves this.
Imo you shouldn't downgrade to nougat. At least based from my experience, it has connectivity issue when i first bougt my Nokia 5 (and still on nougat) the cellular data just connect and disconnect like crazy.
Strange... I have that issue, but with the WiFi. Anyways When I participated in the Android Oreo Beta Labs for Nokia 5, Oreo had a huuugggge battery drain. At the time, there was a way to downgrade to Nougat and it solved all my problems with the battery and thermals. Unfortunately, hoping the general release of Oreo would fix things, I updated to Oreo once again and the battery issue came back.
Also, is there a way to Calibrate the battery? There is apparently a way to do this on the Phone app on Samsung phones so I tried it on my old S5 and it made the battery reading a lot more accurate. I heard you can do the same thing by draining the battery and charging it up again, but ain't nobody got time for that!
If you haven't already, you should consult this using native Support app.
Look, i know i just bought Nokia 5 2 days ago (and now it already updated to Oreo) and i don't see battery drain issue. Using Indonesian unit TA-1053.
My model is different (TA-1024). I tried using the support chat but it says "it depends on usage". That didn't really help at all. The battery was clearly a lot worse on Oreo. On Nougat it later well over 2 days but now I have to charge it at least 2 times a day.
Btw, I really thank you guys for helping me on this. I'm going on holiday soon so I really need that beefy battery life I had before!