So, I think I've found the culprit. It seems that certain services (location, certain apps, etc.) ping the CPU pretty often when turned on, even with WiFi off. Everything off, with Greenify forcing all apps to hibernate (and the device to enter "doze mode"), in battery saving mode, I reduced the overnight charge from 35% to 1%. That's without the device's "auto-shutdown", which Boyue recommended having on (but I didn't like having to boot up my device each time to use it).
I'm not sure if this was a software problem on Boyue's side, but with many of their default Android settings on, the battery drain was massive. Luckily, with some tweaks, it's greatly increased.
I've also been speaking with some devs at Boyue on how to better improve the battery life via slowing down the CPU, so hopefully it will be further improved in a later update. Their customer service has been very helpful so I'm hopeful
