Can someone PLEASE guide me how to get rid of the lousy build-in desktop and switch to MATE_ 

If you join as a maintainer, we could afford to provide yet another optimized desktop, but until then ... one optimised desktop is already hard to maintain. For a generic MATE install proceed this way:

https://www.google.com/?q=MATE+desktop+debian+install

 

Most if not all recipes should work, but there will be no HW acceleration or other "lousy" optimizations. I agree it's not the most beautiful and advanced, but a choice within necessary compromise due to limited resources. Beauty is subjective so it's pretty pointless to discuss which is nicer. IMO both are lousy.

45 minutes ago, Igor said:

If you join as a maintainer, we could afford to provide yet another optimized desktop, but until then ... one optimised desktop is already hard to maintain. For a generic MATE install proceed this way:

https://www.google.com/?q=MATE+desktop+debian+install

 

Most if not all recipes should work, but there will be no HW acceleration or other "lousy" optimizations. I agree it's not the most beautiful and advanced, but a choice within necessary compromise due to limited resources. Beauty is subjective so it's pretty pointless to discuss which is nicer. IMO both are lousy.

Thanks for the answer, I will give it a try. I'm afraid my skill level is not sufficient to join as a maintainer but I'd love to.... Now the look of the chosen desktop is not the main issue (even thou it looks as something the kids has cooked up) but the lack of customization is the main issue. And in my opinion MATE is a much more versatile and by all means a better desktop. Personally I would prefer Cinnamon but it does not work well on ARM...

This is my Spanish guide to add Mate Dsktop to the Raspberry Pi https://raspberryparatorpes.net/raspbian-2/raspbian-mate-instalar-mate-desktop-sobre-raspbian-lite/

But if you start with any armbian Debian Server you can try this simple steps:

 

sudo apt install mate-desktop --no-install-recommends mate-desktop-environment-core mate-themes mate-session-manager xinit mate-terminal mate-applets pluma software-properties-gtk xserver-xorg mate-utils mate-utils-common

Now create a text file with nano

nano .xinitrc

in your home directory containing this:

exec mate-session

And finally add a desktop launcher as you prefer (only one of this) :

sudo apt install nodm

Or 

sudo apt install lightdm