How to run Ubuntu fully in initramfs?
It is not possible using the Ubuntu desktop CD to do this without keeping the USB disk inserted the entire time. However, given that you have a sufficient amount of RAM, you can use the toram casper option to copy the contents of the desktop CD into memory (by using a tmpfs).
Startup Disk Creator will get you most of the way there. Just be sure to use the "stored in reserved extra space" option. Next, edit
syslinux/txt.cfg in the resulting USB disk and put toram just before the -- in every append line except the one for "Check CD for defects."