On this one: It's not really that different. Very little if any KDE code made any direct calls to Xlib or xcb past the very early years. Most of this was also hidden by Qt abstractions, and toward app/shell devs most certainly by further KDE library abstractions. Directly interacting with X11 was an almost-never occurence.

Fairly similar in other toolkits, since most of them have some sort of cross-platform strategy and infra.

People sometimes have kind of odd notions of how exactly X11 development was different. X11 had its fair share of "fragmented implementations" immaturity. It had its fair share of "this new standard is not widely implemented yet" phases that people just don't remember because they started using it after things had settled down (e.g. NetWM and EMWH protocol extensions did not always exist, and also got interpreted quite differently sometimes, or e.g. the clipboard spec). DEs also did plenty of DE-specific things on X11 using proprietary window hints or DBus-based side channel protocols.

If there's one broad philosophical difference, it's Wayland has some widely-respected values around who gets to introspect what state and who acts as authority, that mean that things that X11 apps could monkey-patch may now require a protocol to be agreed on. But there's a lot of value in agreed-upon protocols, and X11 also needed many of them.