Incredibly annoying battery drain/WiFi connected but no internet issue - LG V30 Questions & Answers

My phone has a very strange issue which I'm unable to fix. It happens totally randomly. Sometimes it occurs multiple times a day, then it won't happen for weeks.
It goes like this: Sometimes, even though I'm connected to my WiFi network, apps won't connect to the internet. However, the apps won't give me an error message telling me that I'm diconnected. As the WiFi symbol is active, the phone probably thinks it's online while it's actually not. All apps are affected.
At the same time, the battery drains rapidly (about 10% per hour in standby) and the phone gets slightly warmer. Maybe it is trying to connect all its services to the internet, fails constantly and therefore gets warmer?
This battery drain has also occured in flight mode, so it is not necessarily a WiFi problem. It has happened in various WiFi networks and my other devices don't get disconnected, so the reason must be within the phone.
I've done multiple factory resets but it didn't help. I'm tired of doing factory resets. The only way to temporarily fix it is turning the WiFi on and off again or rebooting the phone. I'm on Oreo btw.
I should add that this only begins when the phone is in standby. When the screen is on, i.e. I'm actively using the phone, I do of course notice the issue but it starts before waking up the phone from standby. That makes me believe it has something to do with standby / Android doze.
Someone on Reddit suggested that it might be due to Android Doze. I followed the advice there, went to Settings > General > Battery > Power saving exclusions, switched on the WiFi and restarted the phone. That was two days ago and there's still no improvement.
Apart from that, battery life is bad overall, even when the WiFi issue doesn't occur. Just see this screenshot from today below. It's almost as if the phone has a bad day sometimes and drains the battery for no apparent reason. Then, on other days, it's better but still nowhere where it could be according to other V30 owners' screenshots.
What can I do?
It is really really annoying!
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Not sure if this would help or not but try (from the playstore) Greenify. It works best if device is rooted but also works (give permissions through adb) without root.
Would it fix the problem? Oh no, doubt that. It *might* point to what's going on though?