100% CPU use doesnt make sense. RAM would be the main constraint not the CPU. Worth looking into - maybe a bug or broken piece of software.
Also the DE may he more the issue than the distro itself. You could install an even more lightweight desktop environment like Open box. Also worth checking whether youre using x11 or Wayland. Its easy to imagine Wayland has not been optimised or extensively tested on something like your device, and could. Easily be a random bug if the DE is pushing your CPU to 100%
There are super lightweight distros like Puppy linux.