However modern FW of PC clones don't only contain a bootloader and a "simple" hardware initializer. Modern laptops have tens of sensors, power measuring parts, power controllers, completely remote management (Intel ME / AMD PSP), embedded controller code for fingerprint, embedded controller code for keyboard etc.
Companies ship a new product line every single year. This churn causes nothing but shitty FW to be developed. Moreover the component manufacturers also slightly change things every single year. That's why sleep sucks with PCs. That's why Fn shortcuts don't always work correctly. That's why batteries drain.
Everything is fixed in the 6 months to a year following the launch in PC industry. Then they start to work on the new model and only supply security updates.
My understanding is Coreboot is a minimum-viable firmware implementation to get your hardware initialized ... and not much else.
Once Coreboot is ready you then use "Payloads".
Payloads can be as varied as booting into Linux directly, GRUB directly, or a whole UEFI/BIOS implementation.
SeaBIOS is a whole FOSS BIOS implementation (commonly used for emulators like QEMU) and is somewhat tractable according to my prior research (~50k lines of C).
My comment about CPU support is due to modern systems using UEFI - the payload for that is TianoCore EDK2. It's quite large (900k+ lines) and while it has much more eyes on it, if minimalism is what you're looking for I doubt it would qualify.
That leaves you with systems from before ~2013 or so - assuming you can even flash Coreboot to them, which only supports a handful of systems.
I suppose you could just jump into Linux though. Perhaps it's silly to do anything else.
Why can't the BIOS/UEFI offer some basic initialisation functionality and then the OS (Linux, Windows, ...) does a more advanced initialisation? Why does it have to be so complicated?