how do I burn a sketch so that it starts directly without a bootloader?
The basic concept is that the bootloader code is loaded into high memory and a configuration switch is set so that power on (or reset) starts at the high address (boot section) rather than low memory (sketch section). This is controlled by a series of microcontroller switches (fuses). To change these, you need an external programmer. To program a board through a programmer, you need a different "boards.txt" entry.
If you read yourself warm and fuzzy on "improved" bootloaders, you may want to know that many (if not all) of these improvements relate to ancient versions of the Arduino bootloader. I really don't think there is much to gain from messing with what you already have. Rather, get yourself a programmer and use that whenever a bootloader is not needed or desired.