> Desktop environment that matches or surpasses Mac OS in quality, performance and UX design.
I've got Plasma setup to take advantage of a decade of Mac muscle memory, and think it does "macOS" better than macOS itself.
> Apps are sandboxed, similarly to Android or iOS
Install firejail, and its wrapper scripts. The wrapper will automatically wrap common commands and apps in a sandbox, and you can use the firejail CLI to launch apps that it doesn't wrap.
I agree with your other points, however I lament that BSD-style licensing will prevent a lot of cool things from having their sources see the light of day or be upstreamed.
> I've got Plasma setup to take advantage of a decade of Mac muscle memory
Care to elaborate or share? I’ve recently moved from macOS to Linux.
Latte-dock with auto-hide for a dock, and custom KWin button arrangements to mimic macOS. A top panel with Window Title + Window Appmenu Plasmoids for a global menu. System Tray and Event Calendar Plasmoids on the same panel, the latter of which has a drop down calendar like in macOS. KRunner, or either the Search or Application Launcher Plasmoids to replace Spotlight. The Screen Edges setting can mimic macOS hot corners. Widgets on the desktop is a compromise for a Dashboard replacement, as is the Launchpad Plasma Plasmoid. Workspaces, Activities and various Screen Edges views can replace Mission Control. Dolphin can be configured to look like Finder, and you can set Finder keyboard shortcuts, too. There's a setting somewhere to make menus transparent and blurred, as well.
Most of these are extensible or scriptable, and have actions that can be triggered with custom shortcuts. KRunner in particular is extensible. Plasma lets you change or set shortcuts for everything, so, for example, you can mimic the Mac screenshot shortcuts well, since KDE's Spectacle is comparable to the latest updates to screenshot functionality in macOS.
Oh, and symlink 'xdg-open' to 'open', or create an alias.