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 itself.
When you want extra control over what has to be sent to the Nextion display, you could consider the program 'NextionDriver' at https://github.com/on7lds/NextionDriver as a companion to MMDVMHost.
This program sends extra information about the host to the display, can do callsign lookup with extended information (name, city, country) and can do more processing which would not be the task of MMDVMHost (for example: with the help of this program, it is possible to use buttons on the display).