Following the Arch wiki, I built a custom kernel for the first time ever. It was easier that I thought it'd be, probably because the Arch wiki is so thorough and because I didn't make that many changes... I just enabled a few FireWire related options as per this guide in order to work on this project (or this).

Results/takeaways

• The OS ran fine with the new kernel 💃
• I learned how to manually create GRUB entries 🔧
• I learned that I don't even have to create new GRUB entries, since I can simply point to the new kernel image and initramfs from the bootloader menu 🪛
• I learned that my 5800X3D runs well (compiles the new kernel blazingly fast) with all cores set to negative 30 in the UEFI
• FireWire is dead 💀

Tiny followup on that project
That old GPU did something to the UEFI (possibly CMOS?), because even after removing all potentially incompatible hardware and reinstalling my modern GPU, I still got that same error message, saying that the system was forces to boot in CSM mode. Only by resetting the UEFI settings was I able to restore the system to a working state. I admit, I panicked.