I'd like to help, but you said Fedora was "vaguely windowsey." I've used Fedora off and on for close to a decade, and I have no clue what you mean. Like, it's not at all windowsey in my experience.

So, in order to avoid what you're talking about in other distros, I'm going to need some more details… what do you mean by "windowsey"?

I’m a huge Debian fan, but I’d say everyone should give openSUSE a shot. It’s a well thought out distro that doesn’t get enough love.

I vote Arch. Yeah, yeah, but it taught me Linux better than any other distro. Yeah you can break it but that's kinda the point because once you've figured out what went wrong you're left knowing how to fix it again in the future.

The Arch Wiki is second to none, is kept updated promptly when things change with the relevant package to the page you're reading and in many cases it's literally referenced as a source for other Non-Arch distro's documentation

Btw I use Arch.

Edit: I also, for no rational reason, would like to be able to use the default package manager exclusively. I mean sure, you can use makepkg manually with the AUR so Arch still applies but that's pretty inconvenient. If you go with Arch and give up this part I recommend yay as my aur-helper of choice.

Debian testing?

It sounds like you want a rolling release distribution. Which means the software repository will have versions close to upstream, or the latest versions shortly after they hit. There's plenty of those.

The best entry level rolling release distribution is, in my opinion, EndeavourOS. It is based on Arch Linux, but provides a fully working desktop out of the box. It grants you a package manager with a rolling release repository.

Next up would be Arch Linux. Similar to above, but it comes with a minimal system and you have to explicitly install most applications that aren't required to boot and start a terminal-only session, including your desktop environment.

Next up would be either gentoo or nixos. But I feel like the other two will cover your bases so I won't talk about those unless you want me to.

Depends.

Do you mind a Rolling Release? If not, try Arch, either Vanilla with the AUR enabled or EndeavourOS. The default package manager (Pacman) + the AUR has pretty much everything you could need without resorting to flatpaks, snaps, or appimages (or, did you mean something else when you said you'd "like to use the default package manager exclusively"?).

If you want something you don't have to constantly babysit, OpenSUSE Leap is a good choice and has as big of a selection as Arch does, or so I heard, anyways. There's also Ubuntu if you don't mind Canonical's Snaps (or know how to get rid of em). You also get the benefits of DEB packages IIRC, but don't know if you'd count them as part of the "Default Package Manager" or not...and off the top of my head, that's it. I'm sure there's more, but none that you can get away with only using native files on it are coming to me

If you open to do some research - you should be able to stay on Debian and use nix the package manager https://hachyderm.io/@fasterthanlime/111099224022408403

Comes with some steep learning curve, almost a learning cliff though. But once past this - it's good.

Just use a rolling release distro and call it a day. You have Arch if you want to get your hands dirty. Or you can run endeavorOS which is almost Arch. You have suse tumbleweed. I have been running endeavourOS for close to two years now and I love it.

Matchmaker, matchmaker, boot me a flash. Find it with "find". Batch me a batch. Matchmaker, matchmaker see the O'Reilly book, And make me a perfect match...

Just use guix or NixOS. Last package manager you'll need

NixOS or gentoo

If you don't mind learning a bit upfront (the installation process is low level but well documented) and want to experience a fantastic package manager, try Gentoo!

LFS

You can make it literally whatever you want at the cost of some time. And you'll learn stuff.

Red star OS

https://en.m.wikipedia.org/wiki/Red_Star_OS