Well, there's some good news. I finally bit the bullet and upgraded the Talos II, expecting I'd just have to deal with Wayland. To my surprise X11 started right up and I'm in Plasma 6.3 with just about everything intact.
But kwin-x11 crashes incessantly on the Blackbird, even with plasma-workspace-x11 installed. I'm pondering if some old residual package is making it work. I would be surprised if the fact the Talos has a GPU is the difference, especially since Wayland does work on the Blackbird with ASPEED graphics.