minimal-efi VS pcboot

If you go the EFI + Clang route, it's a lot easier and you wont need a cross compiler. See my demo here: https://github.com/AndreVallestero/minimal-efi

You don't need a cross-compiler for building bare metal programs for the same architecture. You just need to make sure to pass all the correct flags to gcc, like -ffreestanding, -nostdinc, and so on. Take a look at my pcboot project for reference: https://github.com/jtsiomb/pcboot/blob/master/Makefile

You don't have to. Feel free to refer to my Makefile and linkscript (pcboot.ld) as an example if it helps: https://github.com/jtsiomb/pcboot

There is a more simple way, but it requires direct hardware access, which is something reserved for the operating system kernel when you're running under typical modern operating systems. Basically the graphics card's framebuffer is mapped somwhere in memory, and as long as you're in a video mode (which you can set up in a GPU-independend way with VBE, assuming we're talking about PCs), you can write pixels there and they're interpreted as colors by the scanout circuit. If you want to use the "simple way" you need to write your own bootable program. See my pcboot project: https://github.com/jtsiomb/pcboot, and series of accompanying short articles here: http://nuclear.mutantstargoat.com/articles/pcmetal/

Link to a regular ELF binary with -nostdlib using an ld link script that defines the start address to the address you're going to load at (see mine for example: https://github.com/jtsiomb/pcboot/blob/master/pcboot.ld ).

here's mine from a recent project in case it helps: https://github.com/jtsiomb/pcboot/blob/master/pcboot.ld

You'll be the judge of the "reasonably clean" part, but here's mine: https://github.com/jtsiomb/pcboot/blob/master/src/vbe.c You'll also need to look into https://github.com/jtsiomb/pcboot/blob/master/src/lowcode.s for the int86 implementation, and here's a short accompanying article about how this works: http://nuclear.mutantstargoat.com/articles/pcmetal/pcmetal04.html