What on God's green flat earth are these requirements??

What about apt do you find too complex? I guess what are you defining as 'complex"?

I'm terms of package management you'll be hard pressed to find anything that requires less work that apt, yum, zypper or their various GUIs.

Debian is the most vanilla distro you can get and you are excluding it out of the gate because of apt. So it would be helpful for all of us to understand your complexity issues with apt (and zypper).

It was my first distro and I miss it a lot. Simplicity and stability are main selling points

If you say Debian's and OpenSUSE's package manager are too complex for you, I can tell you that NixOS' package manager is definitely not for you.

Hannah Montana Linux.

This excludes Debian where the package manager is too complex and packages deviate from upstream too much

This isn't even remotely true.

Also going with Debian + GNOME Software + Flatpak isn't a bad ideia at all. Unlike Snaps, Flatpaks are fast you won't notice delays and waste 10GB of RAM for each application you want to use. And at the end of the day you get rock solid Debian + the latest and greatest software as Flatpaks without "deviation from upstream" and you also keep a clean system.

You've picked pretty stupid criteria, but if you're adamant on it, as another commenter said Slackware is probably one of the best options.

Fedora dances with Flatpak quite a bit, but you could double check if RHEL does (since that's what Fedora is based on).

Again, while Slackware (and possibly RHEL) fit your criteria, your criteria seems pretty silly, and you're likely to walk into bigger (and harder to solve) problems on more obscure platforms.

If apt is too complex for you, I don't know what to tell you...

Slackware. It'll take a bit of getting used to but it meets your criteria.

On OpenSuse, sudo zypper install package-name

Not too complex if you ask me.

How is apt too complex? And which is simple then?

Slackware is obvious choice, exactly what you are looking for.

It was my first distro and I miss it a lot. Simplicity and stability are main selling points.

I haven't used it myself, and admittedly am entirely unfamiliar, but a buddy put popos on something recently and was talking about how smooth and simple it was. Might be worth a gander.

OpenSUSE allows minimal installs. Should be possible to go with Leap or the new Slowroll and extend it without all the GUI tools to make your fixed release, KISS system.

What's the complex part about "apt" or "zypper"?

After a hiatus in Mac and windows land, I came back into Linux a with similar wishlist.

It's quite a diversion, but I actually went with FreeBSD. Now it's not Linux but with the separation of base system and packages, you get a stable base that is released at a pretty fixed consistent schedule.

For packages you can pick from quarterly or weekly update schedule, so you can have a stable base OS with bleeding edge software. The binary package manager is easy to use, but if you want more control you can opt for building from source as well.

The init system is BSD based so all main config goes into a single rc.conf file, very easy to understand and work with.

Most mainstream applications such as Firefox, postgresql, nginx etc are just a pkg install away and it natively supports zfs (even as root fs) which was one of the reasons I got really interested in it 10 years ago.

Of course, there is software, especially some younger projects that don't support FreeBSD. So while there are thousands of packages available, some Linux only applications won't work.

Personally, I would pick FreeBSD any time that the software I require supports it. I only run Linux (settled on pop is for now) if the software I need requires it.

Linux mint

Mac OSX

Void linux with its xbps package manager might match with your requirements.

Then solus with its eopkg package manager which I might wait like next year to try it.

Have you tried macos? Seriously though, in Debian you just type 'sudo apt install program,' that is about as simple as it gets. Also opensuse allows you to not install yast with like 2 clicks in the installer, and all the GUI tools are just wrappers for terminal tools that they also include. I am not saying this to be insulting, but I honestly don't think you know enough about Linux to know what you are looking for. If you do want to try Linux, just try a beginner distro like mint.

Edit: upon reading some comments, I see that you mean apt is complicated under the hood, not difficult to use. You could try gentoo if you don't mind long installs. It compiles from source so that is about as simple as package management gets.

It doesn't quite fit your fixed release requirement, but have you checked out void? It's like arch, but has no systemd and it's more stable then arch

Alpine Linux!

NixOS as a daily driver is really good

Just... What? Just stick with Windows

Debian apt repositories is too complex?!?