They aren’t meant to be “flexible”. Immutable means it’s static, read only. You replace one image with another.
In the case of Silverblue you install using overlays, like Flatpak or toolbox/podman.
With NixOS you do get images, but in the form of clojures. BUT it also handles environments on a fundamental level, so you don’t need to reboot to install new system applications or services.
Have you considered Vanilla OS? There’s also uBlue, but I have hopes for Vanilla because it is user-firsf distro, whereas uBlue is more an off-shoot of Silverblue meant for users, but Ruth and same issues as with Silverblue.
Vanilla 2.0 is coming up soon and it seems like a great alternative for people with little to no know-how, or people who don’t want to mess around and find out.