Layout 2 is a no-nonsense layout. It is the original (G4KLX) layout with the Talker Alias added. TA color and fonts size can not be changed. At least not easily.
Layout 3 (as is 4) is a layout without any predefined layout options (color, fonts). It sends the fields *and* information about what was sent to the display, so all layout processing can and should be done in the display ittself.
you could consider the program 'NextionDriver' at https://github.com/on7lds/NextionDriver as a companion to MMDVMHost (it also makes it possible to use buttons on the display).
In verbose mode, this program shows you all communication between MMDVMHost and the display.