Update 1:
CoreElec is available for mecool K5. Unfortunately, dvb tuner is not working so this is only for regular media playback suitable.
Note: all credits goes to the people at https://discourse.coreelec.org/t/mecool-k5-coreelec/13260/33 and also the russian forum https://4pda.to/ where I initially found the reference.
Instructions:
1. Download corelec from website: https://coreelec.org/ or https://github.com/CoreELEC/CoreELEC/releases/
The file to download should be 19.2-Matrix_rc2 (Stable) generic
2. Burn the image file onto usb or sd card
3. Open the usb and go to device_trees and find file sm1_s905x3_2g
Copy the file into main directory
4. Turn off mecool and plug out off power. In order to boot into CoreElec the same procedure must be done as to boot into recovery. So press power on button on mecool and plug again into power. Wait for the splash screen on the tv (my sd card reader has blinking so I know that it is being read).
5. There you should just follow the instructions and recovery method is needed to enter coreelec every time
Update 2:
Also wanted to share how I managed to unbrick my mecool and also to revert to the old initial firmware (0828). After unsuccessful attempts to revert back to initial firmware (to fix the LG remote problem), ended up with a bricked media player. Couldn't boot into recovery, also regular boot wasn't available. To solve the issue, the solution for coreelec gave me a hint.
As before, fair warning that all of this is at you own risk, there is no guarantee that it will work as intended.
Instructions:
1. Download any of the official OTA updates, but they must be in .zip not .img format
2. Open the zip archiver and extract everything into an empty sd or usb card. At the same time put the same .zip file onto another usb or sd card.
3. The same procedure to enter the recovery partition, first put the usb with extracted files into stb, then power it on and it should show you the recovery bootloader
4. Now put another sd or usb card with the zip file into mecool. Go to update over sd or EXT and simply choose the zip file, update
5. If everything goes as planned you should have a working again mecool
Note: for downgrading the firmware you need to have the same version or newer on both usb cards. That is because every time I've tried to update to older version (normal, usual way before bricking the player) it would give an error, saying the installed version is newer.