wlroots and smithay (I'm not familiar with louvre) do a lot of the difficult work for you, that most compositors will do without much variation but there's still a lot that compositor writers still have to do. It's still a significantly larger task than, for example, writing an X11 WM.
(Well, writing an X11 WM that also includes a built-in compositor is a bit more than just the WM, but I'd say still less than writing a Wayland compositor using wlroots or smithay. For example, xfwm4's compositor is around 5300 lines of C, which is... not nothing, but not crazy either.)
(Well, writing an X11 WM that also includes a built-in compositor is a bit more than just the WM, but I'd say still less than writing a Wayland compositor using wlroots or smithay. For example, xfwm4's compositor is around 5300 lines of C, which is... not nothing, but not crazy either.)