Booting iso without USB flash?

[pebcak]

You could boot an ISO from the Grub boot menu. See this post from @manuel for how to add en entry for the ISO:

Is it possible to install a system directly from an ISO without using an installer?

Here is the grub menu entry. You can write it into /boot/grub/custom.cfg, then it won’t be overwritten by any grub update. The entry below assumes the ISO file exists under folder /ISO in the drive where you copy it. Line set isofile=... can be changed if you want to use another folder. Additional note: the menu entry assumes Intel CPU. If you have an AMD CPU, change the microcode name on the initrd line. menuentry 'Boot endeavouros-2021.04.17-x86_64.iso' { set isofile=/ISO/endeavouros-2…

Once booted up you could install to another partition (than the one containing the iso) on the same drive or to any other drive you wish.