Selfhosted

If you are in a position to ask this question, it means you have no actual uptime requirements, and the question is largely irrelevant. However, in the "real" world where seconds of downtime matter:

Things not changing means less maintenance, and nothing will break compatibility all of the sudden.

This is a bit of a misconception. You have just as many maintenance cycles (e.g. "Patch Tuesdays") because packages constantly need security updates. What it actually means is fewer, better documented changes with maintenance cycles. This makes it easier and faster to determine what's likely to break before you even enter your testing cycle.

Less chance to break.*

Sort of. Security changes frequently break running software, especially 3rd party software that just happened to need a certain security flaw or out-of-date library to function. The world has got much better about this, but it's still a huge headache.

Services are up to date anyway, since they are usually containerized (e.g. Docker).

Assuming that the containerized software doesn't need maintenance is a great way to run broken, insecure containers. Containerization helps to limit attack surfaces and outage impacts, but it isn't inherently more secure. The biggest benefit of containerization is the abstraction of software maintenance from OS maintenance. It's a lot of what makes Dev(Sec)Ops really valuable.

Edit since it's on my mind: Containers are great, but amateurs always seem to forget they're all sharing the host kernel. One container causing a kernel panic, or hosing misconfigured SHM settings can take down the entire host. Virtual machines are much, much safer in this regard, but have their own downsides.

And, for Debian especially, there’s one of the biggest availability of services and documentation, since it’s THE server OS.

No it isn't. THE server OS is the one that fits your specific use-case best. For us self-hosted types, sure, we use Debian a lot. Maybe. For critical software applications, organizations want a vendor so support them, if for no other reason than to offload liability when something goes wrong.

It is running only rarely. Most of the time, the device is powered off. I only power it on a few times per month when I want to print something.

This isn't a server. It's a printing appliance. You're going to have a similar experience of needing updates with every power-on, but with CoreOS, you're going to have many more updates. When something breaks, you're going to have a much longer list of things to track down as the culprit.

And, last but not least, I’ve lost my password.

JFC uptime and stability isn't your problem. You also very probably don't need to wipe the OS to recover a password.

My Raspberry Pi on the other hand is only used as print server, running Octoprint for my 3D-printer. I have installed Octoprint there in the form of Octopi, which is a Raspian fork distro where Octoprint is pre-installed, which is the recommended way.

That is the answer to your question. You're running this RPi as a "server" for your 3d printing. If you want your printing to work reliably, then do what Octoprint recommends.

What it sounds like is you're curious about CoreOS and how to run other distributions. Since breakage is basically a minor inconvenience for you, have at it. Unstable distros are great learning experiences and will keep you up to date on modern software better than "safer" things like Debian Stable. Once you get it doing what you want, it'll usually keep doing that. Until it doesn't, and then learning how to fix it is another great way to get smarter about running computers.

E: Reformatting

I don't know anything about what you just asked but man, if there's such a thing as a well formatted post, then this is it!

What's the cost and impact of downtime for you? If you're doing this for personal use it's probably minimal for both so doesn't really matter. If you want to try the new thing and you're not afraid of the time investment or potential downtime then go for it

I don't have any experience with CoreOS so can't help you on that front. That said, it sounds like the server in question isn't mission-critical in the first place and you seem to have come up with a good argument for trying it out. Why not give it a go and see how it works out?

So you've listed some important cons. I don't see the why outweighing those cons. If the why is "I really wanna play with this." then perhaps that outweighs the cons.

BTW on production servers we often don't do updates at all. That's because updates could break, beyond what's expected. Instead we apply updates on the base OS in a preproduction environment, then we build an image out of it, test it and send that image to the data centers where our production servers are. Test it some more in a staging environment. Then the update becomes - spin up new VMs in the production environment from the new image and destroy the old VMs.

Podman runs without a daemon which for some reason makes podman compose an a bit tricky replacement for docker compose.

But for a single purpose, why not just install nextcloud as a system package via layering? I think that should be pretty secure through SELinux and would be the easiest choice.

Other problems with coreOS:

• ignite file make monkey brain confusion
• updates always require a reboot unlike on Debian, where only kernel updates need that (downtime is minimal and can be automated using a systemd service)

pkexec cat /etc/systemd/system/nightly-reboot.service <<EOF
[Unit]
Description=Update rpm-ostree and reboot
After=network.target

[Service]
Type=oneshot
ExecStart=/usr/bin/rpm-ostree --reboot update

[Timer]
OnCalendar=daily
AccuracySec=1h
Persistent=true
Unit=rpm-ostree-update.service

[Install]
WantedBy=multi-user.target
EOF

But I would honestly try it. Maybe give secureblue server a try, should be more similar to your desktop than coreOS (which seems to be made for wide deployments)

Staying on top of updates is one of the most effective ways to keep your stuff secure and really should be done regardless of your setup. Updates have the downside of sometimes causing systems and applications to break. I think the question is what frequency do you want to update your applications.

I have been very happy with FCOS and really view it as building a declarative appliance. You can install it straight from an iso and configure it manually similar to Debian. But I really like the butane / ignition method for defining everything about it. Sort of like a more robust cloud init on the Debian side. I typically define this in a ~~terraform~~ openTofu project and then transpile it to my hypervisor as a vm so I can just keep fine tuning my config until I have it just right. I set weekly auto updates typically and for the most part rarely touch FCOS vms once they are working.