Help me choose a distro, please!

This is asking for trouble.

"Gentlemen, I am new to the country, and I was hoping that you could help me choose a political party."

"I'm looking for a good text editor. What's the best text editor to use?"

"I've heard that various religions have a lot of things going for them. Which religion do you suggest I join?"

Aside from very specialized distros (like, you probably don't want Alpine Linux) most distros will work fine for what you want.

I use this machine for typical home usage: Firefox, a notes app (currently Notesnook), maybe office style tools like word and excel.

Firefox will run on everything. You can definitely take notes on anything, and there are tons of options. LibreOffice will be available for everything.

Steam,

Steam ships with its own set of libraries based on Ubuntu, and stuff targeting Steam will normally use them. It should be pretty distro-agnostic.

Discord

They apparently have a Linux app, which I've never used. The website should work fine anywhere. They have a "deb" or "tar.gz" and don't specify any target distro for either. The deb probably is for Ubuntu, just because it's the most-widely-used desktop distro that uses Debian packages, but I imagine that you've got good odds of it working on whatever. If you want to check, you could just throw a distro on a VM.

I don’t want it to ship with loads of applications; I want to choose and install all of the higher level tools. Shipping with a configured desktop is perfectly fine but not required. Ideally, I can have all of this while still keeping the maintenance low. I think that means a stable OS, a good package manager, stable/automatic updates, etc.

Everything outside of really specialized, oddball distros has package management.

All the major distros that I've used have options to do various forms of a stripped-down install. If you want to install a distro without anything graphical at all, you probably can.

You do have a differing release cycle; I'd probably tend towards a shorter one for desktop use. If you were setting up a ci server that you want minimal interaction with, you probably don't care much about having newer software. But, again, distros tend to have at least options for a LTS release that just gets security updates, even if they have a pretty-frequent set of updates, like Ubuntu.

There aren't going to be particularly "unstable" distros in the sense of crashing. Debian stable is aimed at being software that's passed through multiple phases of experimental testing use and is considered well-tested; it's just their normal distro. There's no pixie dust that makes some distros less-crash-prone. If you're really determined to have more testing, you can use an LTS release, which many distros do but I would not advise for a desktop, especially if you're planning on playing commercial games, which you say you are.

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

You can get open-source software on any distro. Debian is a bit more aggressive than some, turns off non-free repositories by default, but I think that most people turn them on anyway. They also have a separate non-free firmware repository, and I think that most people aren't determined enough to refuse to use non-libre firmware for hardware that they have (though they might choose that hardware with libre firmware in mind). I don't think that there's any distro that is going to ram non-open-source stuff down your throat. Honestly, your largest source of non-open-source software is probably going to be Steam, which you said that you want to use.

I use Debian myself these days. I'm hesitant to argue in favor of distros, because my own take is that the differences (a) tend to change over time, (b) most work pretty well regardless, and (c) I think that few people have actually spent enough time on many other distros to be able to have expert knowledge in their failings (which is something that I've seen in vi-vs-emacs discussions, where I've seen enthusiasts often talk about amazing features while unaware that the other editor can also do the same thing; it takes decades to master either).

If I were picking a "first distro" for someone for desktop use, and disregarding your specific situation, my default is probably Ubuntu. I don't use it myself these days, but it's particularly-widely-used. It has a short release cycle on the non-LTS version (I know that you said you wanted low maintenance, but I've pretty consistently found that one winds up wanting to pull in newer software for desktop systems). It's Debian-based. If one distro gets targeted by a proprietary software package (which I know you also said that you don't care about) it's probably going to be Ubuntu. Aside from past use of Upstart as an init system, it isn't especially unusual. It doesn't require some of the poking around (like enabling non-free repos) that Debian does. It may or may not be where someone wants to be long term, but it's not going to bring a lot of complications. But it's really not going to be drastically better than the other mainstream distros.

Whether that is what one chooses or not, I'd stick to one of the more mainstream distros for a first-time user. There are legitimate reasons to use oddball, young, and specialized distros (tiny, security-hardened, real-time oriented, scientific-computing oriented, music-production oriented) but many of them die out after a couple years or impose constraints that aren't immediately apparent to a new user.

I'd suggest something that's been around for at least ten, preferably fifteen years. A distro that's accomplished that has enough of a track record that they aren't just going to be a flash in the pan; they've been able to attract and maintain enough effort to keep up an ongoing release cycle, which is not easy and I think is often more effort than would-be distro maintainers realize. Most distros that have come out since I started using Linux in the 1990s have died off. If yours gets discontinued, then you gotta migrate off it, which is a pain. But again, if you choose something new and it never sees another release, migrating off it isn't that bad. You're gonna maybe have to learn a new package manager and some new ways of configuring things and new conventions, but most distros don't vary that incredibly much.