1. The main fundamental differences are the package manager, the way the system is setup (partitons, immutable distros), and possibly software you don’t want installed. Aside from that, you can install basically anything on any distro. Some do make it easier than others to install new things though.
2. Canonical (Ubuntu and direct spinoffs) and Manjaro are the ones I recommend avoiding, because their marketing and “general sentiment” goes against my opinions of the distros/maintainers. However, switching Linux distros (especially to another one with a similar base) is not nearly as daunting of a task as switching from Windows to Linux. Some corporate distro owners might pull something like advertising, but there’s often an easy way out (except with snaps).
3. As for the distros you mentioned, Fedora, Mint, and Pop!_OS are all good options. Mint and Pop!_OS are both based on Ubuntu, which could cause issues in the future, but Mint is working on a Linux Mint Debian Edition. Aside from that, my general recommendation is to stay close to upstream. Distros further downstream tend to break more often (think spinoffs of Ubuntu, Arch derivatives, forks of Fedora, etc). There are exceptions to this rule, like when a distro stays close to upstream.
4. In recent times, it should all be working okay! We’re “in the middle of a long time switch” from X11 to Wayland. Those are protocols for the way applications display to the screen. X11 is lacking features, like HDR, and can have issues with “weird” multimonitor setups. Wayland is being actively developed, multimonitor works fine, and HDR is available for some desktop environments (like KDE or GNOME). Not all distros default (or support) Wayland yet, so if you need HDR, pick a distro with KDE or GNOME as its desktop environment.
5. This situation has gotten more complex with Wayland (one of the pain points still being worked on). The features you get partially depend on which DE (or wayland compositor) you choose. Previously on X11, this wasn’t the case. For Wayland DEs, KDE is moving relatively fast, with new features nearly every release. GNOME is moving slower, but should cover most people’s needs. As for tinkering around with your choice of UI/DE, there’s many options available, but KDE offers by far the easiest customization possible (it’s all in the settings menu). There’s more complex, more customizable options available, but I wouldn’t recommend them as a starting point.
6. As for nvidia, it has been progressively getting better, but there are still nvidia specific issues that come up from time to time. There’s not really much you can do about it, aside from following changelogs and updating when the thing you’re running into is fixed.

Now for your list of applications:

• Gaming (through steam) works great! There’s definitely still issues, but I’d argue there’s not really more issues than on Windows, just different issues. There is one category of games that’s still problematic, games with kernel level anticheat. They do not and likely will never play on Linux. Other launchers (EA Play, Ubisoft Connect, Epic) can have their own issues, although there’s often fixes/workarounds available rather quickly.
• Firefox works just fine on Linux.
• VLC works great too, although there are other options available that are more modern or better in some ways. It’s up to you to decide what to use.
• Spotify works just fine, there’s always the website in case nothing else works, but the “app” as a flatpak or even through repos works too.
• Discord has some issues accepting that Linux exists, but have recently started making some changes with that. Most people either use Disocrd in the webbrowser (to prevent too much system access), or run a custom client like Vesktop.
• Godot works great on Linux, I don’t have much else to say about it tbh.
• Visual Studio Code too, it’s basically just a webapp. Some integrations might be slightly different (like the terminal), but otherwise stuff “just works”.
• Git was quite literally made for Linux first (as a project, but also as a platform to run on).
• Photoshop is going to be difficult to get running, if it works at all. You can certainly try, but it might be a good option to find an alternative for this one.
• Audacity works great
• Davinci Resolve does have a Linux version, but the free version can be picky about codecs. There’s always tools to reencode your inputs, but it’s not always convenient drag and drop.
• Misc. tinkering is going to be much more fun, as things in Linux ecosystems are often open source. Not only can you mess around with tools that already exist, you can edit them, or even make your own. Some “niche” hardware might give you issues (like iirc the goxlr, or some capture cards).

https://ntfy.sh/

Easily set up, and easily attached to other things. Simple notifications about whatever is needed, like service health or updates, new posts on public platforms, etc. A simple curl is plenty to send and receive notifications, and it works on Android without requiring FCM (Google infrastructure).

1. A puppeting (personal account) Discord bridge basically requires your own homeserver. You are trusting the homeserver owner / bridge host fully with your Discord account.
2. It is technically against Discord ToS. While I don’t think anyone’s been banned yet, several people have started receiving warnings that they “spammed”, most of them after sending an attachment. These warnings are on your account for 2 years, and could contribute to an account ban.
3. Voice chat is not, and probably will not be supported.
4. Do NOT bridge a “large” server. You are essentially re-hosting the chats, which can be extremely taxing for large and active Discord servers.

I use mine for a single channel in a “medium-size” server (~2k people), a friend group server, DMs, and a few channels that follow a bunch of announcement channels on other servers.