I'm really, really happy with my current setup of Fedora immutable + toolbox [0]. This tool lets you create rootless containers that are fully integrated with the system, so you have acces to a regular mutable system, can install whatever without layering on the base system, run graphical apps, etc. while still having everything inside a container in your home directory. That means no Flatpak required. Highly recommended.
[0] https://containertoolbx.org