The command line can be part of the UEFI boot entry depending on your particular firmware.
I think I can recompile the Linux kernel in around 15 minutes, but I have a 12400F. And 15 minutes is still 60 times longer than most people are willing to wait.
It wouldn't be impossible to change the default kernel commandline in the image without recompiling the whole kernel if anyone cared about making that fast.
I mean, even the initramfs can be stored in the ESP, can't it? How's that work with Secure Boot? (assuming you don't just use a shim that makes the TPM happy and then proceeds to not actually verify anything afterwards.)
I think I can recompile the Linux kernel in around 15 minutes, but I have a 12400F. And 15 minutes is still 60 times longer than most people are willing to wait.