HP ZBook 14u G5 - Power & Sleep issues. Device won't sleep/wake up NOR turn off/on

[im0c]

FYI: I have this ZBook 14u G5 mobile workstation since 2018. Issues I'm describing below, occurred (and still occur) most of the time from back then until today, but now I just had enough of it. There was a couple of months (if I remember correctly - in 2019) that after a certain BIOS update, my device worked normally. Then, after another forced Windows 10 update (possibly interlocked with some automatic HP updates) it went back to "not working as it should". Sadly I don't have any notes on the BIOS version in place back then.

System information:
Product: HP ZBook 14u G5
Total memory: 32 GB
Processor: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
Current BIOS: Q78 Ver. 01.20.00 - 03/21/2022
OS: Windows 10 (up to date)

Issue: 
Device won't go to SLEEP MODE nor POWER OFF/ON "normally" - No matter which method I choose. Be it:

-clicking "Sleep" or "Shut down" from power options;
-closing the lid (if set to sleep in power settings);
-waiting a set amount of time (if set to sleep in power settings);

the device: 

For both SLEEP & SHUT DOWN
1. blacks out the screen but runs the fan like crazy - very loudly - and won't allow me to wake it up. Hard restart is needed (although I'll describe how it's not turning on normally below).
For SLEEP option only
2. "appears" as it went to sleep. It goes quiet and nothing looks suspicious, but never wakes up after (doesn't matter which method I try). Hard restart is needed again. 

1.1
First approach to restart the device: I need to press the power button for about 10-15 seconds (just pressing it once shortly doesn't initiate anything). Then fan stops working and the device seems to be turned off. Now to turn it on, I need to press the power button shortly.

What happens are two scenarios:

1.1.1
Around 80% of times, the device initially appears to start, but it only starts the fan (quite loudly again). Screen stays black and nothing else happens. If I won't turn it off by keeping the power button pressed for about 5-10 seconds, it'll just keep running the fan as is. Otherwise I'm turning it off (or it would seem so).

1.1.2
The other 20% of cases, the device boots just fine.

1.2
Second approach to restart the device alternatively: this funny method works (don't ask me how or why - I have no clue).
-I have to make sure the AC cable is plugged in;
-Now I need to turn off the device by keeping the power button pressed for about 10-15 seconds (as long as it takes for the device to stop working);
-When the device seems OFF, I need to unplug the AC cable;
-Close the lid for a few seconds;
-Open the lid;
-Press power button once; shortly (while device is unplugged)
-Then I can plug the AC cable

I know it sounds like magic, but this is how it works.

2.1
To restart/power on the device, I need to press the power button and keep it pressed for about 20 seconds (just pressing it once shortly, doesn't initiate anything). Then after I release it, the power button led light blinks 1 time and nothing happens. If I keep it too short and the LED light doesn't blink, it won't work. After that, I can press the power button just 1 time briefly and my device turns on properly. 

For POWER ON
3. It's described among above points, because the outcome depends on how the device was shut down. Alternatively (for example after a brief battery removal) it's completely random if the device will power on or not. Sometimes it does, sometimes I need to plug the AC for it to power on, sometimes I need to try couple of times. I haven't outlined any specific patterns for powering on yet.

My current workaround is simply not to turn the device off nor allow it to "go to sleep", but since it is a notebook, you can understand how impractical it is.

Solutions I have already tried:
a) New battery
b) Complete Windows reinstallation (more than once)
c) Some combinations of old Windows 10 and Windows 7 versions (sadly I can't seem to restore/outdate BIOS version to try it too)
d) Not updating Windows 10 after reinstallation
e) Updating BIOS
f) Different power & sleep settings (although probably not all, because I'm not a pro, so if anyone has any ideas on how to combine some of the power plans/settings I'm listening)
g) Different RAM memory cards
h) Windows 11

Any hints (except for "you need to replace your motherboard") will help. Thanks a lot!