in case you are curious, this is the ramdisk i'm booting in my checkm8-based iOS 15 jailbreak:
https://github.com/tihmstar/jbinit
When you reboot the device, all changes to rootfs are gone. I didn't care about persistence yet.
Maybe it's useful for you, enjoy :)
iOS booter ramdisk creator for checkm8 based jailbreaks
Tested on iOS 15.1, should work on all iOS 15
Kernelpatches not included, make sure to bring your own ;P