OK, so I've tried a few things, and had a few surprising outcomes:
I've analyzed the CPU load with various tools, as Android Tuner, OS Monitor, and All-In-One Toolbox.
Most of them reported Android System
using 30% CPU, and com.android.systemui
using 25% CPU, all on the highest frequency.
But All-In-One Toolbox recognized top process instead of Android System as Lockscreen
android process, so maybe there is a hint there...
(and I don't use any custom lockscreen stuff beyond stock CM's one)
So, backed up the current troublesome system (let's call it BKP_NEW), just in case, and decided to restore a full NANDroid backup I had from a week ago (and the issue is just from yesterday).
Restored all 6 partitions (/system, /boot, /cache, /efs, /modem, /data), rebooted, and....
...The issue was still present! Crazy battery drain just as before!
Very confusing... Then I went for a factory reset - it fixed the issue, but I have to start from scratch with all the SW installs and config, and I don't have a week to waste setting everything up again, and potentially running into the same issue some time later.
So, I've decided to move further back, and restored a two week old NANDroid backup > reboot > No issue presents
But it was just too weird, since the delta between 1-week old and 2-week old backup is infinitesimal.
So, out of curiosity I've decided to restore the BKP_NEW (the one of the system with the issue present)... and after the restore it works just fine! (WTF?!?!)
After thinking on it for a while, my conclusion is that there was some problem with some part of system that was wiped/fixed in TWRP's Factory Reset, but is not touched/overwritten during the NANDroid 6-partition restore.
But again, this is just my assumption, need someone more knowledgeable to confirm this possibility...
can advice on this?)
Since that, all is working fine (at least so far).
And if my assumptions are correct, @Ksenios
, you can try to do this, and still have working system with all apps and configs (If youre comfortable with recovery / backups management).