Is it possible to select between X11 and Wayland sessions, just like in Fedora 42 with GNOME on Fedora Cosmic?
No, COSMIC is Wayland only (with Xwayland support for X11 apps).
COSMIC uses the Smithay compositor, which started out as a Wayland compositor and has no plans to add an X11 version.
Do you know if Cosmic supports Espano (text expander) and ActivityWatch?