Just recently repartitioned my MacBook:
1 GB for EFI (vfat)
2 GB for /boot (ext4)
11 GB for swap
224 GB for / (bcachefs)
Grub cannot load a kernel off bcachefs so I need ext4 to bridge the gap. Once the kernel is loaded, it has no problem using bcachefs as root.
This is a laptop. On a desktop that can handle more drives, I would split /home onto a drive of its own.
My current Linux distro uses APK 3 as a package manager. Updates are already atomic without the downsides of an immutable distro.
There are situations where immutable distros make sense but, for my desktop, it feels like a lot of compromise for benefits that do not move the needle for my use case.
Security is also a focus of my distro. My desktop does mot run any server workloads. I mess with it and tinker but I already use Distrobox and a COW file system. And I run two kernels, one bleeding edge (day-to-day) and one LTS (recovery). Recovering from breakage is just not a headline issue.
I guess the other factor is that I have limited time. So, my “tinker” budget is already spent. Playing with immutable distros may change my mind about them but they are far enough down the list that it may be some time before I do.
Bootable containers are something I want to play with though.