It still sounds to me like something's up with the disk. Can't think of any solutions to suggest but I would run a SMART health check on it:
sudo apt install smartmontools
sudo smartctl -a /dev/sda
If you prefer a graphical tool, you can do the same thing with GNOME Disks, which also has options for disk benchmarking.
In the resulting report, the overall health state should be "PASSED", the "Type" column should show "Pre-fail" and "Old age" values, and the "Media-Wearout-Indicator" should be close to 100. If the overall health state is "FAILED", then you will want to back up your files immediately and consider getting a new SSD.
GNOME and Plasma are so far separated that a merger would be impossible, without either eliminating one of the two or completely rewriting both, and I think they cover different niches. GNOME is for people who want a tightly integrated experience, and KDE is for people who want to customize their system. (I would also argue that it's not possible for there to be only one distro or DE, so long as all the components are open-source. Savvy users will always make their own stuff if they're allowed to.)
There's already plenty of cooperation between GNOME and KDE devs on common standards, support for each other's apps, etc. I hope this continues, and makes both desktops better. A lot of behind-the-scenes stuff, like Wayland extensions, could definitely become shared between the two desktops.
You can start anywhere you want! I often recommend starting with Star Trek: The Next Generation, since it's aged a little better than the original series. You might prefer to jump ahead to season 2 or 3 to get to the really good stuff, but even season 1 is worth watching.
Up until Enterprise season 3 it's pretty much all episodic (or in DS9's case, mostly episodic with a subset of the episodes forming a series-long story arc), so you can pick a random episode or movie with a cool-sounding description and start there if you want. That's how I got into Trek, just picking random TNG and Voyager episodes.
Whenever you install or remove software, be sure to read through what's being removed. You don't want to accidentally uninstall something important. This is very unlikely to happen with official Debian packages, but you should be especially careful when installing packages outside of Debian's repo, as they may not be fully compatible with your version of Debian.
In any case, I'd log in to a tty (ctrl-alt-any function key) and install whichever desktop environment you had before using apt.
Debian 12 ships with the non-free-firmware repo enabled by default, including firmware-iwlwifi, but a few Broadcom cards, and maybe others, still require software in non-free if I recall correctly
I'd say it started at a 6 or 7, and grew to a strong 8 over its runtime. Most of the characters have always been beautifully nuanced, but the stakes of its plots have always been unnecessarily inflated, and the endings for each story arc are of very mixed quality. After the jump to the 31st century, the storylines became much more Star Trek-ian, and the show started to display more of its own identity separate from classic Trek and action movie tropes, and that pushed it into properly great territory.
this is my "gaming" Plasma activity, so in theory everything I play regularly is there :)
i'm personally fine with the windows being slightly brighter, but i think an almost-black theme would look good too... might experiment with that, thanks for the suggestion!
Most regular distros are good enough for gaming. The only issue you're likely to run into is with graphics drivers (I recommend going for AMD graphics on that build), and the availability of certain software in certain formats (gaming software is more likely to be available for Debian or Ubuntu based distros).
If you like the Steam Deck's desktop mode, you might enjoy another distro with the same desktop environment (KDE Plasma). I'm partial to KDE Neon, a snappy Ubuntu LTS spin with all of the latest KDE software.
I'm aware of FreeTube and PlasmaTube, which IIRC both require an Invidious instance. There was something called SMTube in the past, not sure if it still exists.
Nothing I'm aware of has both desktop and mobile version, but if anything there are more options for mobile YouTube clients; try NewPipe or Clipious.
Edit: SMTube does still exist. It does not require Invidious, but it does use tonvid.com.
Icon theme is a slightly modified version of Oxylite, only changes are that it follows my system color scheme, the "inherits" list is different, and the start-here and preferences-system icons have been changed.
Wallpaper is Haenau.
I'm using Oxygen for Qt widgets and decorations, with the Obsidian Coast color scheme, and standard Breeze Dark for GTK2.
Layout is entirely my own. I'm showing my Games activity because the main one contains a folder view that might expose info I don't want to expose here.
Hopefully this is original enough? I'm not sure. I've gotten away with posting desktops with mostly existing themes before, but on other occasions I've had posts removed for it. At least I mixed and matched some icons this time.
Revolt is the most Discord-like FOSS chat app; it's very easy to use and customizable. Rocket.chat and Mattermost do similar things and are more oriented toward organizations (the Slack/Teams Classic use case).
/dev/nvme0 is probably your SSD. But if it passed you probably have nothing to worry about