Did you or someone noticed better Batterie Life with Havoc compared to xiaomi.eu roms ?
Of course it's better. MIUI has an insane amount of background services and, moreover, xiaomi.eu is based on a development rom. This means that the rom itself has debug enabled and the kernel too, leading to way more battery usage compared to a standard GSI (which is also compiled as userdebug) but has 1/10th of the background services and, if you flashed OkitaKernel, a kernel stripped of most of the debugging crap.
Edit: also in /vendor/overlay/ remove everything but BluetoothResCommon.apk. The phone will work just fine, VolTE will be magically enabled, network mode display will be fixed and you'll lower a bit the idle battery consumption. There is much crap left in there by xiaomi that is insane.