There's a pending Mutter MR[1] to allow XWayland applications to handle the scaling themselves (which IntelliJ IDEs can do) but it hasn't been merged yet, and I'm probably never gonna see it on 24.04 anyway. Apparently KDE already supports this, but the Gnome folks have been reluctant to adopt the same approach.
I ended up going back to 100% scaling, increasing the system font size, and then setting `GDK_DPI_SCALE=1.25` in `/etc/environment` to tell all GTK programs to increase their scale. It's not perfect, but most things are the right size. I definitely would not have wanted to switch to Wayland any sooner than this though. Transitional periods are such a pain.
[1]: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3567