iPhone 8 Plus 64GB battery drains faster after update to iOS 14.4.2

I own a 2-year old iPhone 8 Plus, 64 GB. I've been putting off the 14.4.2 update for months but I finally updated it one day (at about 10 pm).

That night I went to sleep at 1 am at about 97% but I woke up to its battery discharged to ~50%. Checked the battery usage and found out apps have been running on background. Turned off auto update, auto-brightness, deleted unnecessary apps, restarted the phone.

I was using it the way I normally did and still noticed that the battery still drains faster. That night, I went to sleep with ~40% battery level, woke up to it with ~10%. Normally I can get through a day and a half without having to charge the phone but now I have to charge it within a day.

What do you think is the problem here? I read somewhere that the battery drains faster within 24 hrs of the update, but it's already past that. Please not something about changing the battery because I couldn't afford to change batteries now, money is tight. Would you recommend downgrading to previous iOS?

tl;dr: iPhone 8 Plus still suffers from quick battery drain after updating to iOS 14.4.2, even after doing battery saving techniques