v3 unusable for me

That's mostly what the footnote of my previous comment was about. While I agree it would be nice to have this kind of separation between X clients, I'm afraid it'd be trickier to achieve than it might seem. And it's not clear to me that it'd really be worth the effort. In the long term, most applications under active development should migrate to Wayland native. The circumstances where users need to run multiple apps via Xwayland at the same time should keep getting fewer and farther between. (Multiple Xwayland instances would also result in higher memory consumption, which might matter for some users, if probably not most of them)

> I still want that seamless Wayland <-> rootless XWayland bridging agent though! :)

If a Wayland compositor launches multiple rootless Xwayland instances, it's the responsibility of the compositor to propagate stuff between them as needed. With a single instance, compositors should already be doing what can be done.