I don't know how old your hardware is, but iirc cachyos's kernel specifically doesn't support older hardware, so it could be that it is just too old. If I were you I would try out endeavoros if you can deal with a little more technical package management or you could use the base arch kernel on cachy. Of course your switch to kubuntu should continue to work assuming it was a kernel issue, I just don't like canonicals vision.

The kernels (and accompanying modules/drivers) are more or less freely interchangeable.

Bugs in the kernel are pretty rare in my experience. I think it's more likely that the bug was somewhere in KDE Plasma. Kubuntu's version should be older than the one on Cachy. On top of that Kubuntu has their own patches for KDE, so even if the version numbers are the same they are not the exact same programs. And on top of that the way they compiled KDE will be slightly different.

It's hard to say for sure what's causing the issue, but it's definitely possible that it's kernel related. I've never used cachyOS, but when i was still using Arch i've had issues with newer kernels multiple times. Switching to the LTS kernel would usually fix the problem though. I'm actually facing a weird issue myself right now on GNU Guix, where my pc has randomly rebooted itself multiple times over the past few weeks. My first thought was the kernel since Guix is also rolling release, but i haven't tried the LTS kernel yet because the nonguix substitute servers are having issues, so i would have to wait for it to compile lol.

Oh btw regarding your question, i don't know if cachy offers the regular arch kernel in its repos as well, but generally speaking kernels should be pretty compatible regardless of distro. You can also install the cachy kernel on NixOS for example.

I was experiencing similar issues under Manjaro. I couldn't tell exactly when it happened but attributed it to a kernel or other update.

Are you seeing anything in dmesg? I was seeing kernel ring timeouts just after booting (I don't know how it recorded them from before the freeze but it did). My searches led me to find that the Ryzen 5600 silicon had degraded just enough to be unstable. I could underclock it and it got a bit better but not fixed. In the end I replaced it with a new 5800 and the issues completely went away.

Im having a similar issue on cachyos, but my screen freeze is recoverable by removing the HDMI to external monitor and waiting a bit, then the built in screen will come on and I can plug the HDMI back to continue. I haven't been able to narrow it down I just write down the time it happens and look at the journalctl but can't make sense of it

CachyOS has a handy kernel tool which let's you select what kernel you'd like to run. You can try and play with that.

in your shoes: i would try distros/implementations without wayland or kde to rule those out as possible sources of the problem.

i know that ubuntu will let you switch to xservers, so kubuntu might let you as well.