Not sure if an immutable distro will help you much with that right now. You'd still need someone maintaining Plasma 6 packages for you.

If you're comfortable with Arch, you could use astOS which is essentially Arch on an immutable root filesystem. Or you could use Neon Unstable on a btrfs filesystem with snapshots, and roll back if something goes wrong. But that's all quite convoluted if all you want to do is try out Plasma 6.

I think at the moment Plasma 6 is still for developers who are comfortable enough to get themselves out of the mess. Maybe compiling it and trying it out under a different user account for a while is the best way to test it.