This is still fully possible on Immutable distros (which is why the name is misleading, but unfortunately is what stuck- "image-based" is a better description) and uBlue has a mechanism for it- since they're delivered using OCI containers, it's trivial to fork or derive from the project and add, remove or tweak whatever you need. There's also BlueBuild which is YAML but that's a third party project.

Wasn't aware they were integrating docker-compose, that's the perfect solution. I got so fed up with TrueCharts that I've been considering nuking my NAS and reinstalling OMV or something similar, but I guess I'll wait it out and see what happens.

Seconding this. TrueCharts has been an absolute pain and I would not wish it on my worst enemy.

It does take some adjusting- the pitfalls you'd encounter with Distrobox on Universal Blue are the same as Distrobox on any other distro, so first I'd say to try moving your workflow to Flatpak and Distrobox on your current system or a VM and see how it works out. Generally Flatpak is preferred to a rootless Distrobox which is preferred to a rootful one, but sometimes there's not a Flatpak for something (especially command line tools) and you need access to hardware or system level stuff that only a rootful one can do properly.

Isn't Endless immutable?

Look into the app Repainter. It isn't free and needs root or Shizuku access but does the trick.