Drivers, config you missed/didn't realise was relevant/wasn't needed before, IDs (e.g. disks), etc.

Nix or aconfmgr (for Arch) help.

I still like containers for this though. Scalability doesn't mean I'm fooling myself into thinking hundreds of thousands of people are reading my blog, it means my personal use can outgrow the old old PC 'server' it's on and spill into the new old one, for example. Or that, for simplicity of configuration, each disk will be (the sole disk) mounted by a Pi.

There's more than one way to skin a cat. If you're running something as simple and low profile as OP suggested, all you need to backup from the system are the packages you installed and a handful of configurations you changed in /etc. That could be in ansible, but it could be just a .sh file, really. You'll also need a backup of the actual data, not the entire /. Although, even if all you did was backup the entire / there's a good chance it would work even if you try to recover it in new hardware.

The services metioned by OP don't need to talk to each other, they are all things that work out of the box by just running apt-get install or equivalent. You don't need anything really fancy and you can set up a new box with part of the services if they are ever taking too much resources (which, for a small setup, will likely never really happen. At least in my experience)