It sounds like you’re including NixOS in this category so I guess I have switched.
I also tried Fedora Silverblue a bit, and it seemed to me that ostree distros are built on a cool idea supported by compromises I didn’t like:
Some stuff doesn’t work in Flatpak sandboxing - at least not yet. One example that comes to mind is Firefox integration with the desktop 1Password app. Maybe I could make this work by tinkering with Flatseal, but when install the native packages in NixOS this interaction just works.
I don’t want my CLI tools in a container running a different distro. For example if I’m using Distrobox to set up a dev environment that’s installing a distro with traditional package management to get around not being able to install packages natively in the host OS. I get that Distrobox enables isolated dev environments for different projects. But for that use case I think Nix devshells are more flexible, robust, and performant.
Nix also has its problems - in particular the usual complaint that the documentation is not comprehensive enough to match the complexity of the system.
Since traditional tiling window management hasn’t caught your interest you might check out Niri, which is a scrolling tiling wm. The differences are that windows always stay the size you set them to, remain in the relative layout you put them in, and you don’t fiddle with layout switching. Niri is also especially mouse- and touchpad-friendly. It’s great for pure keyboard use too - you have both options to suit your preference & mood.
I mention Niri despite it not being what you asked for because it checks all the boxes you listed (apart from stacking), and it’s amazing! https://github.com/YaLTeR/niri