Can’t say i can reproduce with the latest ISO, the install, even switch to the unstable branch and then update it to latest plasma 5.25.1 … but llvmpipe was working all the way, no issue whatsoever. The VM install:
What crashes Kwin under wayland session on my end, on that VM install - is starting a GTK application. But normally i do not use wayland, not even on the real hardware … And the X11 session works just fine.
Make sure you are actually on X11 session and not Wayland, especially then forcing it to load like you do.
Now i’m tried with gnome edition, only need use virtualbox without 3D to system automatic use llvmpipe, but any other way to use llvmpipe show the problem, i’m tested in barebone and crash a lot.