Help me choose a distro, please!

I'm going to ignore your "Planned Usage" section. Why? Because that's more-or-less about which software you install, not about the distro (well, not if you choose a well-enough maintained distro at least). If it was a question of family of OSes (windows, mac, linux, BSD) that might be different.

You want Debian, here's why:

I'd like an OS that’s highly configurable

That's most distros

but ships with good default settings

That's Debian. I installed it when i was still a newbie to computers in general, and it hasn't bit me in the ass yet.

and requires very little effort to start using.

See previous answer.

I don’t want it to ship with loads of applications; I want to choose and install all of the higher level tools.

My first Debian was a headless install on my laptop so I could customize the graphical stack. In hindsight, I wouldn't recommend going that barebones unless you actually do take advice and RTFM. I went without a compositor for several years, as an example of why.

Shipping with a configured desktop is perfectly fine but not required.

On the flipside, Debian has GNOME, Xfce, KDE Plasma, LXDE and MATE as installer options. You can also install any Desktop Environment that works on linux, as it is more higher-level software than OS-dependent software.

Ideally, I can have all of this while still keeping the maintenance low.

My other PC is also a Debian (need that on a bumper sticker). It's my daily driver desktop (the aforementioned headless install is a laptop); I set it up based on installer defaults and have not had to do any low-level maintenance on it for the past 2 years that I've had it.

I think that means a stable OS,

Debian is stable af. The downside is that they don't really have bleeding-edge software on the default Stable repository. Testing is newer, and still 99.9% stable, but also not the absolute newest. Unstable lives up to its name, I'm told, but haven't felt bold enough to experiment.

Really though, I'm going to guess that any fixed-release update cycle distro will be as stable as Debian, and any rolling release will be about squashing compatibility issues to make sure you can have bleeding edge software. There are some distros that strike a balance more in the middle of those two, so that's up to your preference and you should probably try out a few before you settle for what someone on the internet says is "The Best." (The main difference between the others and Unstable is that Unstable is a rolling release, instead of fixed)

a good package manager, stable/automatic updates, etc.

Apt is mostly a positive experience. As I mentioned, before, using thr Stable repository will ensure updates are stable and don't break compatibility. I have never had the Pacman experience of not being able to update because there are unresolvable conflicts; the few times I had issues, they were simple enough to fix with a dpkg --configure -a and/or apt --fix-broken install. It can be slow, but frontends like Nala have made that less of a dealbreaker for me.

Last bit. Open source is rather important to me. I prefer free and free.

Debian's core driving principal is FOSS. You definitely can still download and run non-free software on it, and there's even a small section of the main repository that includes non-free sofrware, but the primary guiding principles of the Debian repository are the Debian Free Software Guidelines. Note that these principles are more restrictive than the FSF's definition of Free Software, but the most part there is a large overlap.

Here's a link to the installation page, which includes links to various installers and the installation guide.

The wiki isn't as likely as Arch to come up in searches if you just search terms like "linux [software]" or "linux [issue]", but it's an invaluable resource, almost as thorough as Arch's, and the Debian Project's recommended way for ensuring accuracy to your system.

Finally: I'm going to do that annoying thing nerds online do and tell you that you asked the wrong questions, then answer the questions they claim you should have asked. The linux community as a whole supports and encourages experimentation. You'll find your journey more fulfilling as a whole if you go outside your comfort zone and try new things, do it differently instead of sticking to recommendations and what you know. I know this message is at odds with how much I've talked up Debian, but I was answering the questions you asked.

The truth is that your tools should suit you and your needs and your style of problem solving. All softwares, including the most basic parts of an OS, are tools and therefore benefit from trying different options. Do you want "eh this is okay enough to get the job done" or "this is a fun and fulfilling way to complete projects"?